Opposition to same-sex marriage comes from republicans.

(In a thread on RedState.org, someone took offense to the idea that republicans are "anti-gay," among other things. This post is a short explanation of that perception.)

There's a poll that says 70% of republicans oppose gay marriage (21% approve). Meanwhile, 53% of democrats approve and 31% disapprove. The margin of error is 3.6 for this, so it's possible that democrats don't really quite approve, but it's obvious that republicans really do disapprove.

Independents disapprove (49%) more than they approve (34%) also. Among all polled, 50% disapprove and 37% approve, so the country as a whole is more against it than for it.

Since gay marriage proponents often see this as an equal rights issue for gays (very similar to interracial marriage), they probably see opposition as a sign of a broader anti-gay sentiment. That is, if one supports equal rights for gays, one must also support gay marriage. Therefore, if one does not support gay marriage, one does not support equal rights for gays.

Republicans would say they are not prejudiced against gays, but they still oppose gay marriage. That would be fine but for the equal rights problem. I haven't heard any good explanation of how homosexuals can be considered to have equal rights to heterosexuals (I don't buy the "marriage is already open to all" argument, though I admit it sounds good before analysis).