Be an adult and reply with why you think I'm wrong.
As a white-male-hetero I fully agree with you on an intellectual level. They should have settled for the same privileges with a different name, it makes perfect logical sense... This was my initial stance, one I argued with my LGBT friends in college, and the stance I used to justify not signing their petitions or supporting their demonstrations.
I've come to realize, though, that this isn't a logical issue to them, this is an emotive "rights" issue. Why would they want to settle for the same, while accepting that they are (at least semantically) inferior? Civil unions might destroy the actual discrimination, but the context would still be discriminatory. We'd be telling them "fine, you can marry, but you're still really second class sorry."
Its like if we removed "separate but equal" from African American's, but classified them as different (lesser) still. The actual abuse has been eliminated, but all of the baggage behind it remains.
This is hard for people like me to understand, since I've never really been in that position. I've never been a topic, and no one has ever really wanted to classify me as anything other than the majority. I'm normal, by default. This is a very privileged position.
Another thing that turned me was the fact that I don't actually understand why they shouldn't be allowed to do whatever they want. It doesn't hurt anyone to let them claim the title of "married". It doesn't hurt me. It doesn't hurt society. It doesn't hurt my relationship. If won't make America's abysmal marriage statistics much worse. There is no reason to really oppose it, then, at least according to my strong social libertarian (little-"L") principles. The main arguments I have seen have been from a religious foundation, which holds no water with me, nor should it in government. Some have been based bigoted exclusionist rhetoric; "they are different, therefore evil and corrupting". Or they have been based on naive political ideologies (big-"L" Libertarians, mostly), whose rationals often smell utopian, and verge on being completely aloof of human consequences, and some arguments from this quarter strike me as logically inconsistent.
The big thing is that there really is no reason whatsoever not to let them. It hurts no one. If is a net positive, in that there are more happy people, more inclusive definitions of "rights", and we're closer to all of us having an even playing field. And, a bit more snarkily, it pisses of bigots and homophobes, which is a personal plus.