[Alberta's Child]

. . . that would still potentially allow marriages between, say a father and an adult, adopted daughter.

Which is exactly why such a marriage is permitted by law as far as I know.

Unless gay marriage would curb promiscuity.

Like heterosexual marriage has curbed heterosexual promiscuity? /sarcasm off/

Now that's the kind of argument the gay community really wants to hear -- gay people have so little control over themselves that a legal contract is needed to keep them under control.

[Celtjew Libertarian]

Actually, it's irrelevant in my book whether it curbs promiscuity or not. Back to the original point, it's a question of whether consenting adults will be allowed to arrange their lives as they see fit or not.