Bru, I happen to believe that the government has no business legislating private moral issues. I simply don't see a compelling reason why homosexuality or bigamy should be illegal. These aren't the same as incest and pedophilia and statutory rape and bestiality.
As I see it, the guiding principle behind sexual laws is preventing harm to others. (Incest qualifies because I've read that incest, by definition, involves rape or statutory rape.) As long as the courts adhere to that principle, I don't see much danger of a "slippery slope" in either direction. Once you start illegalizing "deviancy" (whatever the hell that means), where do you stop? Could the slippery slope involve convictions for teenagers pleasuring themselves in private?