First, I believe that we have made marriage a governmental institution by allowing certain rights/freedoms which pertain ONLY to married couples. From tax allowances to the lifting of an inheritance tax for a surviving spouse (which many gay couples are not allowed-- if their partner precedes them in death,they do not have a reduced inheritance tax) to family protections/deductions for married couples and their families... When the government grants these privileges and protections to heterosexual couples and ONLY hetero couples, then I believe it goes far beyond the reach of the church. The IRS upholds the marriage laws, and we know they are a federal governmental agency, not a religious organization. When a church can marry a gay couple and the government does not recognize that marriage or legally uphold it, then this also means, to me, that marriage is not solely a religious institution but a governmental one as well.
Both times I was married, I had to file with the county courthouse and get a license through a government agency. What was not required was any sort of religious officiant of any considerable religious background.
So, no, I do not believe that we should have church=marriage/gov't=civil rights unless EVERYONE were operating under that same umbrella.
I think you are playing devil's advocate in assuming that we have to allow it all-- polygamy, for example, is not a 1:1 contract. That said, I personally have no problem with this as long as everyone involved were participating of their own free will. I have known several polyamorous couples, and while none of those relationships have actually worked out, that really didn't affect me. My concern would be that we had some legal structure in place for sorting out this type of group living arrangement. We'd also have to redefine what family looks like in this context and determine parenting rights, etc.
I trust in my family's culture, how we are raising our son. I do not feel threatened by anyone else's marriage. We have friends who are in a committed lesbian relationship, who were married (which, thanks to Prop 8, was then deemed void) and who are raising a beautiful child in a loving home. I also don't believe that being around homosexuals will *make* my son 'turn gay'. It might make him have a more open mind, embrace a larger vision of what family is, and be a more tolerant adult in the future, knowing he has no reason to feel threatened by another person's sexuality. For me, the only person who can debase or degrade my marriage is me and my husband alone.
So, if other groups have legal contractual protections in place in their relationships, that doesn't bother me. How I raise my son to perceive right and wrong, what is just and what is unjustifiable-- that is entirely up to me. It is not the government's job to uphold my beliefs-- the government's job is to ensure that I have the *right* to my beliefs, even the right to dogma if I choose to --but it is not required to uphold my dogma as *truth*.