There is a HUGE difference between someone who doesn't support gay marriage, and someone who is actually homophobic.
1. A homophobic person HATES gay people.
You can hate what someone is doing without actually hating them.
2. A homophobic can also FEAR homosexuals.
There's a reason it's a phobia. Some people actually fear homosexuals.
3. A homophobic could never have a peaceful conversation with someone who is a part of the LGBT group.
If you're homophobic, you're probably going to insult or hate on homosexuals, and probably wouldn't be able peacefully talk to them as equals. Someone who just doesn't support can.
4. A homophobic wouldn't be able to have a peaceful debate
A homophobic wouldn't ever be able to have a peaceful and respectful debate about this subject. Someone who just doesn't support would.
People who don't support gay marriage get as much hate as homosexuals, and I guess we could argue that it's not a choice because of all the hate we get. But, we don't hate homosexuals, we just don't agree with what they're doing. So next time you meet someone who says they don't support gay marriage, please don't call them a homophobe unless they are actually acting like one.