Gotta hand it to Justice Sotomayor. She asked the most pertinent question about gay marriage of Ted Olson, one of its advocates arguing before the Supreme Court:
Mr. Olson, the bottom line that you're being asked -- and -- and it is one that I'm interested in the answer: If you say that marriage is a fundamental right, what state restrictions could ever exist?” Justice Sonia Sotomayor asked.
Meaning, what state restrictions with respect to the number of people, with respect to – that could get married -- the incest laws, the mother and child, assuming that they are the age -- I can -- I can accept that the state has probably an overbearing interest on -- on protecting a child until they're of age to marry, but what's left?” she asked.
Olson's answer wasn't quite satisfactory. He said banning polygamy would be prohibiting conduct, whereas banning the marriage of gay people is prohibiting their exercise of a right based on their status. But Sotomayor's point (I think) is that you remove the prohibitions for one group, what logical or philosophical argument can you possibye make for not including all groups? (And I think Justice Scalia has made a similar point.) If marriage is a fundamental right for heterosexuals and homosexuals, how can it not be for bisexuals? How can a bisexual find the same happiness as the gays and straights unless permitted to marry one of each sex? Or are we supposed to believe that homosexuals and heterosexuals are born who they are but that bisexuals choose their orientation? Gosh, maybe they're mentally ill, and wouldn't that get everybody's panties in a knot.
I hasten to add that I'm not a fan of the slippery slope argument -- it's considered one of the major logic flaws along with ad hominem aattacks and appeals to authority. We need to argue the merits of the case at hand without cluttering the debate up with what a decision might lead to. It's hard enough to determine cause and effect for actions long in the past, let alone for ones that might happen in the future. But let's not be blind to what we might be opening up here.