Well done.
Note, this coworker in question didn't say "I don't think gay marriage should be an institution of the state" or some other impersonal statement that can actually be evaluated through logic. There is a distinct difference between those two types of statements. Do you think it's reasonable to defend people who say "I don't like gay people."?
As long as he don't act out on it, and go around killing gay people or something.
Just like you have the right to not like George Bush, or football jocks, or goths, or whatever. Nothing wrong with that. As long as you don't try to assassinate George Bush, you have the right to not like him.
I mean, sure, not liking gays make him a homophobic, but as long as he don't continuously go around calling all gays "Faggot" to their faces, harassing them, threatening them, etc.
Good call. As long as your opinions remain "acceptable" you have nothing to worry about.