If you only want to accept particular unlawful acts that you agree with, then of course others will have other unlawful acts that they agree with. Considering how many different groups have how many different sets of values, that road leads to anarchy.
Have we not seen enough anarchy in Haiti, Rwanda and other places to know not to go there?
The last refuge of the gay marriage advocates is that this is an issue of equal rights. But marriage is not an individual right. Otherwise, why limit marriage to unions of two people instead of three or four or five? Why limit it to adult humans, if some want to be united with others of various ages, sexes and species?
Marriage is a social contract because the issues involved go beyond the particular individuals. Unions of a man and a woman produce the future generations on whom the fate of the whole society depends. Society has something to say about that.
Even at the individual level, men and women have different circumstances, if only from the fact that women have babies and men do not. These and other asymmetries in the positions of women and men justify long-term legal arrangements to enable society to keep this asymmetrical relationship viable -- for society's sake.
Neither of these considerations applies to unions where the people are of the same sex.
Centuries of experience in trying to cope with the asymmetries of marriage have built up a large body of laws and practices geared to that particular legal relationship. To then transfer all of that to another relationship that was not contemplated when these laws were passed is to make rhetoric more important than reality.