The reason gay marriage is such a hot button issue is that, for gosh sakes, gay sex is icky.
Of course, I used to think the same about broccoli, sushi, chinese food, red wine and oral sex. But then I grew up.
But there are those who gosh darn it just DON'T want to even THINK about icky things. Lucky for them the Bible has some pretty harsh language to back up their beliefs and feelings, giving them the legitimacy of the word of 'God' to back up their intolerance and closed-mindedness. (Never mind the inherent hypocrisy of using this one part of Leviticus and ignoring the rest. That's a whole 'nother topic.)
For people like this the concept of banning gay sex is a dog they can hunt with. However, since even the religious right realizes that they can't pass laws regulating what consenting adults can do in the privacy of their own home, all they have to fall back on is to deny people who engage in such behavior any claim of legitimacy in the law. Thus the banning of gay marriage.
They love to couch it in terminology that is Rovian, Luntzian in it's seeming legalese. "Protection of Marriage," and "Protection of the Family." Hey, who wouldn't be against something like that? Except that this is not about protection, but about persecution, about discrimmination.
The legal aspect of marriage can and should be administered and enforced by government. The moral aspects of marriage should be solely up to the individuals concerned. Never the twain should meet. This is separation of Church and State and the most intimate, personal and powerful level. Who will you have in your bed? What will the two of you do there? And what the h**l should Focus on the Family, Bill Frist, Arlen Specter, Rick Santorum, Dick Cheney, George Bush, Congress or the Supreme Court have to do with *any* of it?
All because some people think it's icky.