Let me ask you a question...
How does it offend you for gay people to marry?
I can explain how it offends them when you say what you say though.
"Well I don't think we should be keeping them as slaves anymore, obviously we should just let them go about their business, but I really get offended at the thought of them going to our schools."
"Well I don't think we should keep them from having the same rights as married couples anymore, obviously we should just let them go about their business, but I really get offended at the thought of them joining our club (marriage)."
I'm pretty sure you'll find something in the constitution about seperate but equal not being equal even if it might have sounded really nice at the time. After all, we stopped making them pick our cotton, the least they could do is stay out of our schools.
Obviously I don't mean to say that denying gay people the right to marry is akin to the travesty that slavery was, but the point is you want to make them "almost" equal to you, but not quite and we (sadly) have a very recent memory in our history of another group we did this too.
Full disclosure - I'm a married, straight, white, male who isn't offended by them joining my club.