I'm sure that rw considers this kind of sideshow to have no more to do with the "historic institution of marriage" than shacking up does.
Both are a bad joke.
One of the canards often trotted out in discussions such as these is the demand for perfection. Unless one side can show absolute perfection -- all marriages are perfect all the time -- the other side says that marriage isn't necessary, important, valid, et cetera.
Until the Supreme Court started messing with marriage big time in the 60s, marriage in American was virtually ubiquitous, with around 95% of the adult population married, and a divorce rate in the single digits.
Today's proponents of "free" love don't know what they're missing from when the entire culture supported marriage and people's homes and houses of worship were the center of the universe. It was wonderful. Many of us miss this very much, and mourn that our children and grandchildren are losing out. It wasn't just a culture of "me-me-me" all the time, but one of the community and the group and the nation, and what would be good for most people most of the time.