Let's legalize polygamy and incest then. Let's stop dancing around this and choosing one or two taboos to vindicate, let's throw away the concept of taboos entirely.
From a principled standpoint, I'm more Libertarian.
If six people want to shack up together in a house and sign various contracts sorting out their powers of attorney and other such things... that's their business. The government shouldn't try to set up special tax statuses for people based upon who they live with or whatnot and where for, and it shouldn't try to get involved with who is allowed to knock up who (or to fornicate with whom) among consenting adults.
While distasteful - if people want to engage in bestiality - I do not believe it is practical policy to grant animals legal rights and protections under the law, and they are simply choosing what to do with their property (though from a systemic standpoint - laws such as that should be left to the states - I'm simply stating what I would suggest my state's standpoint should be).
Obviously, since children are not consenting adults, then it would be a violation of the law to engage in *** with someone who is not able to give legal consent.
Beyond that - the Government should stay the hell out of it. If a church or community wants to recognize a couple... or... trio... or whatever as married, that is their business.
Of course - I also don't believe we should have a personal income tax that is used to try and incentivize marriage and other such things by the nation.
I may not -approve- of homosexuality, but that does not mean I believe it is the government's place to get involved in marriage or sexual relationships.
Of course, the fact that I can separate my personal views of right and wrong from what are correct and incorrect systemic policy makes me an unusual person. Most people simply base their views of laws and governing policy on what their personal views of right and wrong are.