Gay marriage bans don’t make sense for a lot of reasons, but I’ll just focus on one for now. The notion of defining marriage as a union between “one man and one woman” doesn’t work unless you define “man” and “woman,” and we actually don’t have airtight definitions for those states. I know. You think biology has all the labels and definitions you could possibly want. But sex is complicated.
Edit: I continue this discussion with a follow-up post: Clarification, Sex Determination, and Cheesecake