Legalizing gay marriage can bring up interesting discussions about the socially engineered "nuclear family," i.e. how we institute a legal framework for traditional western social engineering and why. I think it's an interesting policy discussion as to why we as a society promote "traditional" marriage through various laws such as the tax code, and I think it is theoretically possible to raise a legitimate question regarding "why gay marriage but not others." Obviously, too often (or all of the time, really) it's not an honest question (e.g. Huckabee) and is merely a quick political soundbite absent of any actual critical thought.
Gay marriage neatly fits in with our current social engineering framework: two parents, one generation of kids in the house, with everyone splitting up when the kids get older and forming their own new households. Rinse, repeat. The nuclear family, (the modern idea of it, at least, without cemented gender roles) which is still the most basic family unit for a variety of economic and social reasons, remains intact with gay marriage.
Polygamy and incest don't fit that mold. Again, I am completely casting aside moral or "eww factor" considerations. I'm even ignoring Constitutional issues here--I know what a shocker. To address the social engineering/public policy question as to why gay marriage is different from polygamy and incest, I can think of a few (obvious) reasons. Polygamy is associated with male ownership of females or the conception of females as property. Males traditionally have superior property rights. Wealth becomes hoarded in continually closed families. Incest has similar and worse problems: health concerns, family sexual abuse concerns, male dominance concerns, and again the hoarding of wealth and the discouragement of social mobility, which the nuclear family is designed to encourage. From a public policy perspective there are a variety of perfectly legitimate government interests in not promoting these family structures our not recognizing them, that do not apply to gay marriage.