Marriage, in my view, certainly has a religious component, but civil marriage has been approved by society outside of the relgious context for quite some time. Laws usually contain a moral component, reflecting a consensus of society’s views.
No one seeks to control (at least not anymore) the actions of consenting adults, but no one has a “right” (as that term ought to be understood) to have society put a seal of approval on any relationship they choose to have.
Bingo. Who you marry is no one's business, as long as you're both consenting adults. All you need to do is sign a standard civil contract.
The nanny-liberals want the state in on this because it breaks down resistance on other fronts. Make no mistake, the goal is not equality. It is control.