How to argue against gay marriage (part III)

And now for the rebuttal.  In the last post I said that if I had to make an argument against gay marriage--in a debate club setting, or some such--then perhaps I'd say this:

1. There's a great deal of value in knowing both of your biological parents and being raised by them (other things being equal--of course there are exceptions).
2. The normal way of procreating for same sex couples involves a gamete donor, so that the child knows and is raised by at most one biological parent.
3. If same sex couples are permitted to marry, their normal way of procreating will be elevated to parity with the heterosexual way of procreating.
4. Because of that elevation to parity, we will all gradually lose sight of the value of knowing both of your biological parents and being raised by them.
5. Losing sight of a kind of value tends to cause that sort of value to be less pursued and less protected. Possible sequelae: couples (straight or gay) buying gametes to have the type of child they prefer (an intellectual, an athlete, or whatever); rights of biological parents being less protected.
6. There isn't the same problem with infertile heterosexual couples being able to marry, since using a gamete donor is not their normal way of procreating, but usually a last resort chosen after years of medical intervention.  THEREFORE,
7. Same sex marriage should be prohibited (but marriage of heterosexuals who may turn out to be infertile should be allowed). 

I think the premises of this argument are either true or quite possibly true (I'd say 1, 2, and 6 are true; 3, 4, and 5 might be true).   But the conclusion doesn't follow.  The argument makes an error that's described eloquently by Jonathan Rauch, in a debate with Maggie Gallagher.  The error is to think that any risk of bad consequences justifies denying gay people marriage rights.  If that's what you think, then you implicitly give no weight to the rights and preferences of gay people.  Listen here.  He makes an extremely important point when he says

It is not good enough just to say "gay marriage might be bad for someone else, therefore all these gay people don't have it." In fact, every life is important, and those gay lives must be weighed in the balance.

There's no quick step from procreative risks to denial of marriage rights. The risks, even if they were considerable, would have to be weighed against the rights and legitimate preferences of gay people themselves.

Even if a sizable and certain risk could be established, we'd have to explain why procreative dangers ought to stop us from letting same sex couples marry, while we allow very old men to marry young women (danger: offspring who quickly lose their fathers); and we allow "little people" to marry (danger: high risk of progeny with dwarfism), etc. etc. We grant everyone the legal right to marry who they wish, and normally take it as a private moral question who should procreate, and how. 

Well, it was the best I could do.  In the comments to the last two posts people suggested some other bad arguments against gay marriage--bad arguments better than some of the usual fare.  At least none of our suggestions involve the idea that once you allow gay marriage, you've got to let people marry their cats.  The opponents of gay marriage typically seem not to be satisfied with making bad arguments; they like to go for completely atrocious.