the better question is why not allow polygamy as wellIn order to take advantage of the tax code, inheritance, health insurance benefits, etc. Do proponents of gay marriage also support the "right" of straight people, who may not even live in the same state or even know each other well, to marry each other specifically to avail themselves of the incidents of marriage?
Does the current law prohibit two people of the opposite sex to get married specifically for the advantages you cite?
nope, and i'm pretty sure you don't have to perform a sex act in front of the clerk here in mass to get married if you happen to be two people of the same sex, either.
as long as all parties are consenting adults