No, strictly speaking you don't have to be homophobic to be against gay marriage. Not, if your objection is based on a reason.
But if you ask what they reason is many will say that god said it was wrong. Now as an atheist that doesn't carry much weight with me, but it makes a difference if they honestly believe it.
So is that the honest reason? It could be for some.
But here's Fred who hardly goes to church at all, but spends his time drinking, and fornicating and coveting his neighbours ass. He puts down his glass long enough to say "it aint right- the bible says so".
Do we think his opinion is based on sincere belief and obedience to the will of god?
How about Jane who is against all gay people and likes to quote Leviticus, but who has no problem with wearing mixed fibres etc. How sincere is that?
Too many people look in the bible (or the koran) to find a reason for the hate/fear they already feel.