While mirror image thinking about gays is fun (they're just heterosexuals with a bit flip), it's not all that realist.

Before we get to adoption, lets analyze an easier statement: Gays shouldn't be allowed to become Catholic priests.

What? Can it be defended?

Well, you've heard about the abuse cases. Two-thirds of the problem, going by published numbers, involve priests who have sex with young boys. These are not children, but rather teenagers. There can be argument about whether homosexual sex with a pre-pubescent is really homosexual behavior, but there is no argument about post-pubescent sex. This is a problem with gay priests.

At most, gay priests make up 30% of the Catholic clergy. More likely, they are in the 10-20% range. The male population is 3% homosexual (yes, I know you heard 10% and that you should drink 8 glasses of water a day and that we only use 10% of our brains; the 3% number for male homosexuality has the advantage of actual facts to back it up). So gays make up an out-sized proportion of Catholic priests and there are various reasons for that. Guilt, etc., of course, and a homosexual seminary culture that is now being rooted out by Catholics who (somewhat wrongly) blame it for lots of things that are wrong with the Church.

Regardless, the gay Catholic priests seem to be committing a far out-sized proportion of the abuse. The Church simply can't afford to keep paying out the damage settlements that it would have to in order to keep its gay clergy. The congregations don't like the clergy having sex with their young boys either. Hence, expect to see less gay priests going forward.

While many gay men make wonderful priests, a few ruin it for the rest of them.

So, adoption? While a person could make arguments about equality and fairness before the law, etc., I personally have a low tolerance for that sort of democratic cant and tune it out. It's illogical rabble-rousing. As Socrates says (I'll point out that I'm a fan of the Symposium -- yes, that's the Platonic dialogue on love with a disturbing amount of gay sex thrown in), democracies equate things that are not equal.

Throw those arguments out -- which I do, and with prejudice -- and you have to make a case for choosing a segment of our population (homosexuals) that is more likely to do drugs, have psychiatric problems, etc., etc., and singling them out to allow adoption. It's not a brilliant move.

Aside: The percentage of the female population that is homosexual is harder to get a feel for, but somewhere around 1.5% seems to be exclusively homosexual. Bisexual behavior is much more common in females, or so Kinsey found. The weirdness of homosexuality in humans, a weirdness found in only one other mammal, so far as I know, is that opportunistic homosexual behavior. That happens all the time in the animal kingdom. It's exclusive homosexuality that is non-standard. That's bizarre from a fitness standpoint. There are some electrode studies (electrodes + dirty pictures, you imagine the rest) that suggest that there are no genuine bisexual males as far as desire goes, only males that are bisexual in behavior. It's harder to tell with females (30% or more are unaroused by dirty pictures of either sex, which ruins the studies).