David Velleman continues to surprise me with his posts at Left2Right. Now he's defending the following view. Other things being equal, children should have two parents, one male and one female, both of whom are their biological parents. Now other things often aren't equal, which is why adoption is not really second best for some children and for some parents. One reason is because "knowing one's biological relatives provides a kind of self-knowledge that is not readily available otherwise". There's more, but that's a good part of it for him. For some children, adoption is better than all other options, and for some parents adoption is the only option. However, David thinks an ideal world would have all children raised by their biological parents.
Then he admits that this has some relevance to the issue of gay marriage, though it's not as simple as saying that this makes gay marriage something less than heterosexual marriage. He doesn't think that at all. There's something about gay marriage that can't achieve something heterosexual marriage tends to have (though not for all). That's the ability to have children who are biologically connected to both parents. Of course, as with adoption, other things aren't always equal, so this doesn't affect adoption by gays either. Ideally, though, parenting by gays on this view whould include one of the couple as a biological parent of the child, other things being equal, which of course is often not the case.
Interestingly, that result stands in tension with the other thing David points out. He thinks this is a part of the reason many conservatives are against gay marriage. If so, he says, then it makes sense to oppose anonymous sperm and egg donors, which conservatives don't seem to be saying much about. I do remember people getting mad when this sort of thing started becoming common, but it's sort of become accepted among conservatives. David's point is that conservatives opposed to gay marriage on the ground that a child should ideally have two biological parents, other things being equal, should also oppose the practice of donating sperm and eggs. As usual, it's a fairly provocative argument, and the issues it raises are worth thinking about even if you disagree with his starting point.