Why wage war on certain people's genitals?

In all the heated Prop 8 arguments that seemingly exploded from every point in the universe simultaneously, there's one idea I hardly ever see mentioned, despite its crucial importance. Perhaps it's never mentioned because it's simply too obvious. I'm referring to the fact that disallowing gay marriage is legal discrimination against certain people because of their genitalia. The official reason that two men cannot be married in most states is because they both have penises. Likewise two women and their vaginas. For all the fuss and foofaraw about religious freedom, freedom of speech, rights, benefits, and tradition, it simply boils down to genitals.

And why? Why should we care so much about what kind of private parts a married couple has in their pants? Why do the parts become not-so-private when it comes to marriage? Is it society's business to inquire about such things, or even further, to ensure that every state-sanctioned blessed union comes with one snail and one oyster? If so, why?

One group of people that is profoundly affected by this policy, but also seems generally forgotten in the public discourse, is the community of people whose gender identity is ambiguous or has changed. Specifically, intersex and transgendered people. In fact, I personally have a friend who was living as a girl when I knew her in high school, but he is now living as a man and is engaged to be married (someday) to the woman he loves. He and his fiancee seem very happy together, and I am happy for them. I have not specifically asked my friend whether he now has a penis, whether he formerly had a vagina, or whether his sex organs are or were ambiguous. The reason I haven't asked is because it's none of my fucking business. Yet from what I understand, our society has taken up the mantle not only of investigating my friend's genitals, but of making it very, very difficult for a transgendered person to be married to anyone.

Again I ask, why? What useful purpose does it serve to forbid certain marriages because of sex organs? If society (somehow) benefits by hiring a bouncer to check people's underwear at the chapel door, does that outweigh the benefit of two people's pursuit of happiness and equal protection under the law?

As usual, the Onion hits this point much better than I ever could, so enjoy this video from a year or so ago:


Conservatives Warn Quick Sex Change Only Barrier Between Gays, Marriage