If we denied gay people the right to vote or denied them the right to own property or denied them the right to refuse to quarter soldiers, you might have a point.
WHERE is a government recognized marriage a right?
Interesting that you compare the two. It's a perfect example that illustrates just how wrong you are, legally speaking.
We don't have a right to vote, what we have are a series of protections to prohibit discrimination against various groups when it comes to exercising the power of voting. If you let group A do it, group B and C get to too.
California's Supreme Court has already held that somewhere in their State Constitution (something like a equal-rights non-discrimination amendment or clause, I forget the details) applies to gays and it applies to marriage, ergo, if you allow group A to marry group B and C get to too. Since Prop 8 is effectively revising the document instead of just tagging onto it they're going to shoot it down. Prop 8 supporters are going to need more than a simple majority if they want to get the State Constitution properly amended to prohibit gays from marrying.
Let's say it doesn't go down in flames though. What's next?
Probably the right to marry within the family, something I fully support if we're going to have gay marriage. Put away the pitchforks. It's not like that.
The reasoning behind most laws against marrying your cousin or sister is the slightly elevated risk of various genetic diseases. Some states actually allow you to marry a 1st cousin if you're unable to have kids, like the male has gotten a vasectomy, or you're both over 65 years old. They're state laws, so they're different, but that's the general theme.
So, if two dudes can get married why can't I marry my brother?
Stop looking at me like that. This is a thought exercise!
There doesn't have to be any sex going on, but I can see why this would be beneficial to old guys that are unmarried. Tie the knot with your brother and your insurance covers him, it's easier to share assets, and in my state (MI) we'd be able to trade handguns back and forth without any stupid paperwork or registration hassles.
Next, with it firmly established in our society that you cannot discriminate based on sex, why couldn't a dude marry his sister? Cousin? I could see a legal argument for it. It'd probably be shot down, but somebody would try exploring it.
I'm not trying to make a slippery-slope argument here or anything. I'm fine with gay marriage, just exploring the unintended legal consequences.