If we redefine marriage to allow gay marriage, then we need to redefine it to include anyone who wants to marry. Polygamy, incest, it's all fair game for state sanction if we allow gay marriage.
Agreed that we should allow people with other preferences to marry.
What's the problem? What is wrong with Polygamy?
it is easy to throw in stuff like Incest etc but thats looking for generalisations IMO,