Old joke, worth repeating:

Q. What does a lesbian bring on a second date?

A. A U-haul.

Q. What does a gay man bring on a second date?

A. What second date?

I was reminded of this gag last month when the American Academy of Pediatrics issued an upbeat report on same-sex parenting, and then again a few days later when Quebec's Justice Minister announced plans to give same-sex couples full adoption rights. Every time I hear people boosting "same-sex" adoption, I immediately ask, "which sex?"

As a group, gay males are at least an order of magnitude more promiscuous than lesbians. This fact is relevant to the adoption debate because infidelity has a destabilizing effect on households. It leads to what the experts euphemistically call "negative outcomes" for children -- parental separation, parental death from AIDS, emotional trauma and domestic tumult. For this reason, it is unfair to compromise lesbians' claim to adoption rights by lumping them in with men.

The link between male homosexuality and infidelity is well-supported. In a massive study reported in The Journal of Sex Research in 1997, Australian researchers asked more than 2,500 homosexually active men how many sex partners they had over their lifetime. The most popular answer category among older men was "101 to 500." But "501 to 1,000" and "more than 1,000" each accounted for more than 10% of responses.

In the United States, two pairs of investigators -- gay themselves, for what that's worth -- found that sexual fidelity is virtually unknown in the gay male community. "Of 156 couples in one study, only seven, none of them together for more than five years, had had a totally exclusive relationship," reported a doctor who reviewed the data. "The other study concluded that 'the cheating ratio of "married" gay males, given enough time, approached 100%.' "

In lesbian partnerships, by contrast, infidelity is relatively rare. And, as with heterosexual relationships, it is generally covert and episodic. That should not be surprising: Thanks to a variety of powerful evolutionary factors, women -- whether gay or straight -- generally steer clear of anonymous sex. This is true even when they are given the encouragement and opportunity to act promiscuously. Consider Toronto's "Pussy Palace," a bathhouse conceived in 1998 as a place where lesbians could meet for casual sex and thereby bond behaviourally with their gay male counterparts. The premise was that women would frolic sexually in the male style if only they had a suitable venue. "A lot of women have not had the freedom to explore their sexuality," an organizer told the local media last year. "Young girls are not taught that their sexuality is theirs, for their own pleasure."

Predictably, the Palace bombed. Women showed up, but most preferred to keep their clothes on and talk. Canada's biggest city can support dozens of spots where gay men meet for anonymous sex, but apparently not even one for lesbians. As U.S. commentator John Derbyshire wrote last year, "the conflation of lesbians with male homosexuals is mainly squid ink. The two groups have precisely one thing in common: They are both romantically attracted to their own sex. In practically every other characteristic, they are not merely different but opposite."

What I find surprising is how scared people are to talk about the gay gender gap: Researchers and politicians -- including the authors of the above-mentioned APA study, the government of Quebec and other Western jurisdictions that are moving to permit gay adoption -- generally insist on treating "same-sex" couples as a generic group. They treat as taboo any suggestion that lesbians should enjoy rights that gay men do not.

Unfortunately, this blind spot is part of a larger problem: the thoroughly superficial state of academic research in the area of gay parenting. In recent decades, dozens of scholars have presented research that they say proves there are no differences in developmental outcomes between children raised by same-sex parents and those raised by heterosexual parents. But a scathing analysis published in the American Sociological Review last year found these researchers frequently ignore evidence at odds with their politically correct theses, and that "ideological pressures constrain intellectual development."

Even the unbiased studies are mostly worthless. "Because researchers lack reliable data on the number of lesbigay parents with children in the general population, there are no studies of child development based on random, representative samples," write the American Sociological Review authors. "Most research to date has been conducted on white lesbian mothers who are comparatively educated, mature and reside in relatively progressive urban centers most often in California and the Northeastern States."

While the experiences of monogamous lesbians from Vermont and San Francisco are certainly worthy of study, I would suggest to the American Academy of Pediatrics and the government of Quebec that they have little to say about whether gay men should be permitted to adopt children. While the case for lesbian adoption appears strong, the case for gay male adoption is weak. And it will remain that way until unbiased, male-focused research proves otherwise.