I would like to begin by thanking Larry of Rust Belt Philosophy for participating in this debate. Gay marriage is an important and topical issue and I'm glad for the chance to have my own thinking challenged and stimulated. In my opening post I will present four arguments against gay marriage. (1) the logical coherence argument, (2) The adoption argument(3) the tacit confession argument, and (4) the unhealthy role models argument.
The Logical Coherence Argument
I believe the strongest argument for gay marriage is that consenting adults have the right to marry. Advocates say, "You may not approve of homosexuality, but that does not does give you the stop gays from marrying. It is an injustice and an act of bigotry to offer marriage to straights but not to gays. It is a violation of the basic principles of liberty on which this nation was founded." So let's start by taking down that argument. If marriage is right for consenting adults, then it needs to be applied to all consenting adults. That includes couples in incestuous relationships and polygamy.
Progressive typically object that this commits the logical fallacy of the slippery slope. But if belief A logically entails belief B, then it is not a logical fallacy to point out that A entails B. As Aristotle points out, man is a rational animal. One of our first intellectual duties is to hold our beliefs consistently, and that includes accepting the logical consequences of our beliefs. Progressives who use the "marriage is a civil right" argument face an unhappy choice. (1) Get postmodern and deny the intellectual duty to be logical, (2) hypocrisy, or (3) abandon the argument that marriage is a civil right. I think that (1) and (2) are equivalent to conceding defeat and yielding the public square to their opponents. So that leaves progressives with option (3).
What progressive typically do is create some sort of essentialist argument for gay marriage. There are a variety of ways to do this. For example, William Saletan argues that polygamy shouldn't be allowed because jealousy kicks in when there is more than one other partner. That's an incredibly weak defense because jealousy is a response to the needs of heterosexual procreation. Men are concerned about the paternity of their children, and women are concerned about having a provider for the long haul. The best essentialist definitions of marriage take us back to traditional marriage. (Note that I reject these essentialist arguments for a variety of reasons that should be clear to anyone who frequents philosophy blogs - is/ought, appeal to nature, ...). And I have to reemphasize the point: essentialist definitions requires rejecting the "marriage is a civil right" defense. Instead only certain types deemed worthy of marriage. We don't have essentialist arguments for freedom of speech. A neonazi's speech is not essentially good, but he still has the right to speak.
The other approach is to bite the bullet and accept the fact that legalizing gay marriage logically entails legalizing polygamy and incest.
The Adoption Argument
Adoption is our best window into the moral principles that are relevant to gay marriage. The ethical rules governing adoption have been in place for a long time and they are largely untainted by ideology. The single most important ethical rule about adoption is that it is unethical to pay the birth mother. The reason is due to the first rule of economics: incentives matter. If adoptive couples pay the birth mother, then some women may have children simply to make money. Why would this be unethical? Because it involves intentionally bringing a child into the world for the premeditated purpose of separating it from its biological parents. The ethical rules of adoption recognize that doing this is wrong. Well, that is exactly what gay couples do when they choose to procreate. They bring a child into the world with the premeditated desire of separating it from a biological parent.
Some libertarians take the "markets in everything" approach too far and advocate for a market in babies. Poor women get to make money and infertile couples get babies. Everyone wins, right? Wrong. This does not consider the fact that babies have a vested interest in being raised by their biological parents. One of the most basic principles from evolutionary psychology is this: investing time and energy into one's own biological children is a good strategy, but investing time and energy into someone else's children is a bad strategy. There are some exotic exceptions to this general principle, but overall it is a striking and consistent pattern of behavior in the animal kingdom. It also applies to humans. Martin Daly and Margo Wilson are evolutionary psychologists who wrote the book Homicide to put this principle to the test in humans. They found that children are 70 times more likely to be killed by a stepfather as their own biological father. They did not test this for gay couples, but the simplest and most parsimonious explanation is that this pattern holds for people in same sex relationships as well. I've noticed that when lesbians break up, the biological mother almost invariably gets custody.
I am using homicide simply to illustrate the general principle that children are safest and most secure when they are attached to their biological parents. The alternatives impose a real risk upon children, and it is immoral to do this. That is why the law recognizes the fact that infertile couples do not have the right to have children. This is true even though adoptive parents are carefully screened by neutral third parties and can thus make a strong guarantee that the child will be well cared for. Similarly, many gay couples can provide a caring home for their children, but ethically it is just as wrong. It does not matter if a couple is infertile because the man doesn't have enough sperm, or because his partner is another man. They both do not have the right to intentionally bring a child into the world with the intention of separating the child from its biological parents.
The Tacit Confession Argument
The tacit confession argument holds that there is no need to even get one's hands dirty in the empirical research on gay couples. That is because all the studies have tacitly conceded defeat. How do they do that? One method is by comparing heterosexual couples who go to sperm banks with lesbian couples who go to sperm banks (Chan, Raboy, and Patterson 1998). Of course, this guarantees that you will not compare lesbians to married heterosexual parents raising their own biological children. We already know that heterosexual marriage does not work as well when parents are not raising their own biological parents. See also the point above, about stepfathers being 70 times more likely to murder their stepchildren.
The second major method is to compare children conceived by sperm donors with some artificial measure of American families. We see this in a recent study (Gartrell and Boss, 2010) which finds:
According to their mothers’ reports, the 17-year-old daughters and sons of lesbian mothers were rated significantly higher in social, school/academic, and total competence and significantly lower in social problems, rule-breaking, aggressive, and externalizing problem behavior than their age-matched counterparts in Achenbach’s normative sample of American youth.
We see two ways that the study avoids a direct comparison. The first is that it uses Achenbach's normative sample of American youth, but if you scroll down in the study, you find major differences between the parents in the sample and the lesbians. The second method is the lesbians conceived via sperm donors. That means that their children had elite genetic material. Once again, a direct comparison is avoided on two separate fronts.
The Unhealthy Role Models Argument
Many defenders of gay marriage have a very heteronormative view of homosexuality - they believe that gay people are just like straight people. That is not true. Gay couples are different in other ways besides sexual attraction. The research shows that a gay men in long term relationships are substantially more likely to have open relationships. The research on lesbians is a bit mixed.(LaSala 2005). In fairness, the research also shows that, unlike heterosexual couples, open relationships do not destabilize the relationships of gay couples.
However, this may well create problems for the children of gay relationships, particularly if they are straight. Open heterosexual relationships are not very stable or healthy. The irresistible force meets the immovable objects. Gays often need their relationships to be open, but straights almost invariably need them to be closed. So accepting gay marriage means accepting (1) unhealthy role models for straight children, or (2) redefining marriage into something that is temporary, even for straights. Divorce rates for straights have approximately tripled since 1960. They could go up even higher for children raised by gay parents. Defenders of gay marriage often ask "how does my marriage threaten yours?" But the better question is "does my open marriage threaten the marriage of my children?" The answer is likely to be "yes."
