I'd like to make one thing clear: being against gay marriage does not (necessarily) equate to being homophobic.
I am an atheist so there is no religious angle from me, however, here are 3 reasons why, based on simple human biology, why gay marriage can never be the same as heterosexual marriage:
1, Consummation. A man and a woman who have not consummated their marriage can have it annulled (this applies to EVERY body regardless of religious belief). This is not the same as a celibate marriage being invalid legally as a marriage at all, it's just that one of the marriage partners has the right to end the marriage because of no sexual intercourse. But if both parties agree and stick to an agreement not to have sex, then they're still married. And sexual intercourse means just that: penis in vagina intercourse.
For true equality, there must be a consummation aspect to same-sex marriage. Or, consummation removed from current marriage legislation. Simple as that. So, all the talk of 'my marriage won't be affected' is nonsense. Heterosexual marriages will be affected.
2, Adultery. Adultery is specifically defined as sexual intercourse. In order for there to be equality, homosexual people will need this defined in law. Which, of course, cannot possibly be the same definition as it is for heterosexual people.
3, When a child is born to a married couple, the husband is automatically assumed to be the default other parent. Impossible with gay people.
So, you see, when it all comes down to it, all this talk of 'equality' as in things being the same one-size-fits-all marriage is utter rubbish.
And this has nothing to do with god, religion or homophobia AND everything to do with marriage law.