The Journal of Law & Family Studies (Univ. of Utah) has published a paper entitled Gender Complementarity and Child-rearing: Where Tradition and Science Agree.  They review current literature on hetero two-parent, single-parent, and same-sex parenting and outcomes on children.  Of course, these guys are from Utah, so you might assume they have a pro-family, anti-gay bias.  However, their paper is worth reading, and full of scientific references. 

The article discusses at length homosexual parenting and adoption, and though it has some of the "classic" arguments against gay and lesbian adoption, it has lots of data that we should not ignore just because it is unpleasant.  While this entire post is free of moral judgements, I’m sure someone will find the data "hateful", or some authors as less than trustworthy due to some reason or other, but the data itself is meaningful.  As my old scientific mentor used to say, "All data means something – it may not mean what we want it to mean, but it means something."

1. Fatherless children exhibit what psychologists call "father hunger"

What are the consequences when fathers are not present? Alfred Masser, a psychiatrist at Northside Hospital in Atlanta, Georgia, noted that more and more children who seek psychiatric help are suffering from father-hunger (1989). Blankenhorn (1995) concluded that father-hunger is the primary cause of the declining well-being of children in our society and is associated with social problems such as teenage pregnancy, child abuse, and domestic violence against women….

The paper also summarizes some findings that we are all familiar with, but are downplayed (for obvious reasons) by pro-gay lobbyists and scientists:

2. The APA is now a political tool of gay activism, and is causing psychology to lose credibility because it is less about science and more about advocacy.

In spite of the overwhelming evidence citing the importance of mothers and fathers to the healthy development of children, attempts have been made in professional literature to blur the lines between genders, and to claim that neither mothers nor fathers are necessary for positive child outcomes. Such research reports have become increasingly bold with their activist agendas. Perhaps the boldest of such articles appeared in the flagship journal of the American Psychological Association in 1999. In the article titled "Deconstructing the Essential Father," the authors argue that "neither mothers nor fathers are essential to child development and that responsible fathering can occur within a variety of family structures"

3. Studies show gay parenting worse for children, but often, gay researchers leave that data out.

The claim has been made that homosexual parents raise children as effectively as married biological parents. A detailed analysis of the methodologies of the 49 studies, which are put forward to support this claim, shows that they suffer from severe methodological flaws. In addition to their methodological flaws, none of the studies deals adequately with the problem of affirming the null hypothesis, of adequate sample size, and of spurious non-correlation. ….

Likewise, Williams noted that Lewis found social and emotional difficulties in the lives of children of homosexual parents, but such data did not seem to find its way into her conclusions.
4. Gay parenting produces more gay children, supporting the environmental, gender identity hypothesis.

Lesbian mothers tend to have a feminizing effect on their sons and a masculinizing effect on their daughters. The bigger question is how healthy is the rejection of gender roles? Gender non-conformity is probably the only factor in the literature that predicts future homosexuality.
5.  Studies definitively link childhood molestation with later adult homosexuality.  Is this merely correlation, or causation?  (Me thinks the latter.)

Studies on sexual molestation and its relationship to homosexuality have mental health consequences. Shrier and Johnson (1988) found that homosexually assaulted males identified themselves as subsequently homosexual seven times more often as the nonassaulted control group. In half of the molestations, physical force was used. The mean age at which the molestation was reported was 18.2 with a range from 15 to 24. The mean age at the time of the molestation was from four to six with mean age of 10. Of the extension group, "one half of the victims currently identified themselves as homosexual and often linked their homosexuality to their sexual victimization experiences"

The researchers found that 46% of the gay men and 22% of the lesbians reported homosexual molestation in childhood. In the compared heterosexual group, the homosexual child molestation rates were 7% of the heterosexual men as compared to 1% of the heterosexual women. The researchers noted that this was the first study to report substantial homosexual molestation of girls.
6. Homosexuality correlates highly with behaviors associated with mental illness.

Now, I’m not sure what this means, but it should be investigated.  Is it because they are social outcasts, or because homosexuality itself is unhealthy?  If we can’t ask and pursue these questions scientifically, then we’re no longer doing science, but politics.

Among the sample as a whole, there was a distressing high prevalence of life events and behaviors related to mental health problems. 37 [percent] had been physically abused and 32 [percent] had been raped or sexually attacked. 19 [percent] had been involved in incestuous relationships while growing up. Almost one-third used tobacco on a daily basis and about 30 [percent] drank alcohol more than once a week–6 [percent] drank daily. One in five smoked marijuana more than once a month. Twenty one percent had actually tried to kill themselves, more than half had felt too nervous to accomplish ordinary activities at some time during the past year and over one-third had been depressed.
7.  Gay and lesbian homes suffer more domestic violence.

Not only is the higher risk of mental health problems important to consider when thinking about placing foster kids in gay/lesbian homes, what about the fact that gay and lesbian households have a higher rate of domestic violence than hetero homes?

High rates of violence in lesbian and gay relationships finds significant support in the research. In a study Lockhart (1994) found that 90% of lesbians surveyed had been recipients of one or more acts of verbal aggression from their partners during the 12 months prior to the study. Thirty-one percent reported one or more incidents of physical abuse. Lie and Gentlewarrior (1991) found that more than half of the lesbians had been abused by a partner. Island and Letellier (1991) noted that the incidence of domestic violence among gay men was almost double that of the heterosexual population.
8. Male homosexual promiscuity linked with a slew of diseases and a rise in HIV infections in the U.S – looks like education isn’t working.

A second article by Gross (2003) contained the ominous warning: "The Second Wave Will Drown Us." Gross cited the Centers for Disease Control (CDC) statistic of a 14% increase of HIV/AIDS among homosexual men in the United States between 1999 and 2001. He noted unprecedented outbreaks of syphilis and alarming rates of rectal gonorrhea. He also noted an emerging visible subculture of "barebacking" (men who have anal sex without condoms). Gross concluded that "behavioral interventions to promote condom use–the only strategy currently available to stem the MSM epidemic–are failing".  (Admittedly, this data is a few years old, but is the same thing still going on?)….

The following diseases have extraordinary frequency among gay men: anal cancer, chlamydia trachomatis, cryptosporidium, giardia lamblia, herpes simplex virus, human immunodeficiency virus, human papilloma virus, isopora belli, microsporidia, gonorrhea, viral hepatitis types B and C and syphilis. The transmission of some of these diseases are so rare among heterosexuals as to be virtually unknown. Other diseases, such as syphilis, were found among heterosexuals, but not nearly as prevalent as in the gay population (Diggs. 2003). The CDC (1999) reported that 85% of syphilis cases in King County. Washington were among gay men. Presently, syphilis has reached epidemic in San Francisco (Heredia, 2001). Besides diseases, physical conditions associated with anal intercourse include hemorrhoids, anal fissures, anorectal trauma and retained foreign bodies (Barone, 1983). The list continues with the "gay bowel syndrome" and extremely high rates of parasitic infections (Hastings & Weher. 1994; Kazal. 1976).

Although the study of medical conditions associated with female homosexuality is relatively new, bacterial vaginosis. Hepatitis B, Hepatitis C, alcohol abuse and intravenous drug use was significantly higher among lesbians than among heterosexual women (Fethers, 2002). In one study of lesbian women 30% had bacterial vaginosis, which is associated with high risk for pelvic inflammatory disease and other sexually transmitted infections (Berger & Kolton. 1995).
9. Monogamy and fidelity much rarer in gay relationships than heteros

Perhaps the most extensive study on sexual fidelity was conducted by Michael, Gagnon, Laumann, and Kolata (1994), who found that the vast majority of heterosexual couples were monogamous while the marriage was intact. Ninety-four percent of married couples and 75% of cohabiting couples had only one partner in the previous 12 months. In contrast sexual fidelity is so rare among gay men that a new term has been offered: "Monogamy without fidelity." Gay men who were coupled reported that they had sex with some one other than their partner in 66% of their relationships during the first year, rising to 90% if the relationship lasts for five years. In one study, 15% of gay men and 17.3% of lesbians had relationships that lasted for more than three years.

Another extensive study on homosexuality and monogamy was conducted by McWhirter and Mattison (1984), focusing on evaluating the quality and stability of long-term homosexual couplings. The study was actually undertaken to disprove the reputation that gay male relationships do not last. The authors themselves are a homosexual couple, one a psychiatrist and the other a psychologist. After much searching, they were able to locate 156 couples who had been in relationships that had lasted from one to thirty-seven years. Two-thirds of the respondents had entered the relationship with either the implicit or the explicit expectation of sexual fidelity. The results demonstrated that of the 156 couples, only seven had been able to maintain sexual fidelity. Furthermore, of those seven couples, none had been together more than five years. In other words, the researchers were unable to find a single male couple capable of maintaining sexual fidelity for more than five years.

The expectation for outside sexual activity was the rule for male couples and the exception for heterosexual couple. Heterosexual couples lived with some expectation that their relationships were to last "until death do us part," whereas gay couples wondered if their relationships would survive (p. 3).

A more recent study published in the journal AIDS (2003) found that gay relationships last 1 1/2 years on the average and had an average of eight partners per year outside those relationships.
Believe it or not, that is just a little bit from this very long article.  If you finish that article, you can also read Review Of Research On Homosexual Parenting, Adoption, And Foster Parenting (PDF).