Depending where you are and how old you are, it's already happened.

Same-sex marriage has been legal in Canada for 7 years now, since 2005. My wife's a high school teacher, and when it was legalized her students were 8-9 years old -- too young to care about who can and can't marry whom. Same-sex marriage being legal is, to them, now the default. They don't see it as being a 'debate' and and don't really understand why it's such a big deal in the States.

She says that homophobia in general is almost entirely a thing of the past. There's the odd student that says something homophobic, but all the other students look at them like they're weird. This is a complete 180 from when I was in high school 8 years ago.