Do polygamy bans violate the natural rights of bisexuals? In wake of the current Supreme Court debate in the US that gay marriage bans violate due process and equal protection guarantees, I want to ask a philosopher whether these two legal concepts, due process and equal protection (which go by different names in different countries), are derived from natural philosophical rights. If so and assuming that they are similar in meaning, does that mean that at least philosophically speaking, polygamy irrespective of particular examples is NOT inherently immoral?
The main philosophical argument for gay marriage from what I've heard is that since sexual orientation is a fundamental and largely unchangeable part of a person's nature, it is immoral to deny gays a right that straight people have. But what about bisexuals? Isn't a bisexual woman or man who is in a serious relationship with both a man and a woman at the same time just as deserving? I don't think it matters whether or not the other two members of the triad are intimate with each other so long as they consent to the marriage as well. But if it does matter, is this the sole point of debate?
I ask this as a philosophical question and not a legal question, as I realize the latter must address manipulative purposes of marriage from fundamentalist religious groups, tax dodgers, swingers, et cetera.