Look at the progress we have made with women's rights and minority rights over the past several decades. 50 years ago a white person and a black person in my state couldn't marry. Less than a century ago women couldn't vote. 2 centuries ago black people were slaves and were considered Three-Fifths of a human being.
We move forward. We're slow, but we get there.
Gay marriage will be no different. Just like racism must be taught in order to be learned, so must intolerance and homophobia. As younger generations take over our country they'll care less and less about whether or not gays and lesbians marry. It will eventually become a non-issue.