Homosexual Marriage and Civil Rights

In a previous post I offered an argument defending heterosexual marriage without using any religious rhetoric.

Homosexual advocates are saying heterosexual marriage-only amendments and laws are the new civil rights fight. I want to momentarily sidestep the cries of civil rights violation and ask the question:

Is there any absolute good/bad line that can be drawn regarding sexual activity?

Is there any sex act that Society overwhelmingly considers wrong?

What about adults and minors?
What about adults and very young children?
What about adults and animals?

The problem is once Society agrees there is at least one act that is wrong and it should be prohibited, you open the big ol’ can of worms labeled "violating a person's civil rights". And that is what they call a slippery slope. The civil rights argument starts falling apart fast once you agree there is a Group X that must be prohibited from an act.

So who determines what is good? Some argue "anything that does not harm others". Some would argue "anything that benefits the State". Some believe it is the Judeo-Christian rules.

Why accept these so-called religious rules?
1) They were given to us by a Creator smarter than us.
WHOOA DUDE – You can’t use religion in this argument
2) Regardless of where these rules came from – God, man, or alien - they have withstood the natural selection of societal evolution.

The question remains: does only allowing one male-one female marriages discriminate against homosexuals?
What about the discrimination against polygamist? What about a heterosexual couple unwilling to marry, why should they miss out on the perks of marriage either?
Because one male-one female marriage is in the best interest of the State. (See An Argument For Heterosexual Marriage)