An oft-heard argument against gay marriage is that it will hurt traditional marriages by loosening the ethic of monogamy among married heterosexual couples. The reasoning goes this way:

Premise (1): gay men are more promiscuous than either straight men, straight women, or lesbians;

Premise (2): married gay male couples will therefore be more promiscuous than straight couples or lesbian couples;

Premise (3): the non-monogamous behavior of gay male couples will, by their notorious example, weaken the monogamous commitment of married straight couples;

Conclusion (4): which will hurt and destabilize traditional marriages, with all manner of harmful consequences for children and for marriage as an institution.

Promiscuity and non-monogamy will have spread from married gay couples to married straight couples (and even married lesbian couples?) like a deadly, transmissible avian flu decimating whole families and moral codes that come into contact with it. (I'm only slightly exaggerating this argument for effect.) Let's call this the contagious-promiscuity argument.

Is there any reason to think it's plausible? Certainly if Premises 1-3 are correct, then Conclusion 4 is right. And if the conclusion is correct, gay marriage would indeed cause some harm. We should be very concerned if heterosexual marriages become more non-monogamous than they already are. It might cause so much harm, in fact, that it would more than offset the large individualistic and modest communitarian benefits that I argued for on Monday and Tuesday. If that's true, gay marriage should be rejected no matter how important it is to gay families. It would certainly not be a cause that any traditionalist should embrace.

But it's not plausible to believe that married gay male couples will spread non-monogamy to marriages between men and women. Here's why:

1. Problems with Premises 1 and 2

Premises 1 and 2 are at most half correct, and even the half that's correct is often wildly exaggerated. They're only half correct because they leave out married lesbian couples, who will probably be half of all gay marriages and may be even more monogamous than married straight couples. If it's fair to use the presumed non-monogamy of gay men in the argument as if it's some kind of contagion, it should also be fair to use the super-monogamy of lesbians as an inoculation against this presumed contagion. If gay men will set a bad example, lesbians will set a good example. Why are lesbians almost never discussed by opponents of gay marriage? Why do they not count?

Even as just a claim about gay men, Premises 1 and 2 are usually overstated. I have already argued, in my post on the magnitude of the benefit last night, that claims about hyper-promiscuity among gay men are empirically unsupported. They are based on untutored prejudgments about gay men, anecdotes, and junk science. There are differences but they are not large.

Further, as I also noted in last night's post, even to the extent that there are differences, those differences can be attributed somewhat to the fact gay men have been denied the social encouragement of monogamy through marriage. Marriage itself should help reduce any moderate differences that already exist, even in gay-male marriages where there is no woman to encourage it, weakening Premise 2. Also weakening Premise 2 is the fact that traditionally minded and monogamously committed gay male couples will be the ones most likely to marry, reducing the moderate differences between gay men and others at the outset.

2. Problems with Premise 3

We come to Premise 3 (gay male marriages will loosen the ethic of monogamy in heterosexual marriages), then, with an already weak set of predicates. For Premise 3 to be correct, straight married couples will have to overlook the example set by super-monogamous married lesbian couples in order to follow the moderately more -- and declining -- non-monogamous example of a few married gay male couples who might be openly non-monogamous. By itself, this makes Premise 3 very dubious.

But there are yet three more reasons to doubt Premise 3.

First, there is no reason to believe that heterosexuals look to homosexuals as role models for their own sexual behavior. Indeed, many heterosexuals seem to define their lives almost in opposition to what they see as the gay "lifestyle."

Second, women will always be present in straight marriages and women will for the most part continue to demand fidelity. Jill is not going to agree to let Jack live it up because she heard somewhere that Adam and Steve down the block are swinging from the chandeliers.

Third, as some commentators on this blog have repeatedly noted, we are talking about a very small group of people. (Gay marriage is very important to gay couples, but not terribly important to most others.) If married gay male couples were going to be, say, 50% of all marriages, we might expect their behavior to have some effect on the way the rest of married couples view the importance of monogamy in marriage. But that will certainly not be the case. It's not reasonable to think they will loosen the ethic of monogamy for everybody else, even assuming they are uncontrollably and openly promiscuous and that access to marriage will not change that.

To see why the truly troublesome gay male marriages will be such a small number, let's do some math.

As I noted on Monday, homosexuals are probably no more than 3% of the population. (Many conservative critics of gay equality argue that the number is even lower than that, perhaps 1%.) Gay couples will likely get married at a lower rate than the general population, at least at first, so gay married couples will likely represent less than 3% of all marriages. Male married couples will be even rarer at first. The experience of Vermont civil unions shows that twice as many lesbian couples as gay male couples get hitched. Half of gay marriages would be lesbian -- and they will be super-monogamous.

The potentially problematic gay couples -- the gay men -- will therefore represent perhaps 1.5% of all marriages (using assumptions most generous to the contagious-promiscuity argument). Some of them will manage to be faithful all or most of the time, so the truly troublesome unfaithful gay male couples will probably represent less than 1% of all marriages. Of these non-monogamous gay-male marriages, some portion of them will be very discreet about it, not wishing to incur the disapproval of their families and friends. Thus, the notoriously, openly, innoculation-resistant gay male couples setting a bad example for everybody else will likely represent somewhere around one-half of 1% of all marriages in the country. That's 0.5%. And I think that's probably high.

These paltry numbers will undermine the institution of marriage? Undermine it more than the large percentage of married people who already acknowledge in surveys that they have been unfaithful? Undermine it more than married straight couples who go to swingers' conventions and troll websites devoted to non-monogamous sexual liaisons among married people? Undermine it more than the super-monogamous married lesbians will help it?

Which is more likely: That the 0.5% of openly non-monogamous married gay male couples will exert an irresistible gravitational pull on the morals of the 99.5% of all the other married couples? Or that the 99.5% will exert tremendous social pressure on the recalcitrant 0.5% to change their ways?

I'll get to some better arguments against gay marriage soon. Next up, the polygamy slippery slope argument.