"Widening Marriage"...
This is an example itself. In order to "widen marriage" you are still working within the framework of laws, the government is blessing these things and from the government all things flow.
I am on the side of negative rights... The government doesn't have a right to mess in this stuff, unless there is a victim. (Always ask first, "where is the victim?") If you cannot find one, then the government's role should also be "none". The "weaker party", in what way? What exactly do you mean? Folks can sign contracts, and contract law can apply, so long as both parties are informed and there is no coercion, but that has nothing to do with Marriage.
As a social libertarian I say: The government should not be involved in this at all, unless there is a child involved as they are not old enough to give consent, or if all parties involved are not informed because then you have fraud, but a list of government approved relationships should not exist. Laws that protect victims should exist, not ones that define marriage, folks have religions for that and they can follow or not as they believe.