Roommates and best friend can get married/civill-unioned. If they do it because one has good health insurance through their job for example, that's better than one of them mooching off of the system, but I don't see that happening. It hasn't happened with male-female couples racing to the courthouse to tie the knot before the judge. Undoing it can be too expensive and time consuming, and the fact that (in most states) both parties have a 50% equity in each others' property would prohibit casual unions in homosexual couples just as they currently do now in hetero couples.
As far as the son with the mother, being that they're immediate family, I don't believe there'd be any reason for them to do that, and it's also illegal in most states for one to marry one's mother/son.