I'm not too sure what you mean by boundaries, exactly, but I'll give it a shot.
Basically, similar benefits to a marriage. We are joined legally in the eyes of the state, we recieve similar taxing, we are considered on the same level - not the same page - as a married couple.
However, I see problems inherent with this idea. What about gay christians? They'll surely still push for a true marriage. This may sound selfish, but that is not my agenda, not being a member of the christian community and having no interest in it's workings. Religion has put a wall between christians and homosexuals, and I will have no part in it. As I've said before, I'm not out to upset the traditions of your religion - I'm out to start my own set of traditions.
Sorry if this seems dodgy and doesn't answer your question. If you wish, I'll give it another try when I have a clearer head.