Katherine Hepburn used to say that; “it’s just not possible” to roughly quote her. It’s silly; but having said that, I can’t stop two people who think they’re “in love” from shacking up together. I doubt they’d be inviting me to their “wedding” anyway if it became “legal”.
No, that's right, and neither can the State of North Carolina, for example. If any association of people other than one man and one woman want to "get married," they can find a church that will do it for them, or get a Wiccan practitioner, or a friend with some cool academic robes, or whatever. What they can't do, without the state's police power behind them, is force me or anything else to recognize them as a married couple.
I had a friend from college who held a "commitment ceremony" with his homosexual partner. We were invited, although it was a courtesy invitation, since we lived in a far-off city with several small children. It was a housewarming celebration, as well, so we sent them a gift to recognize their home-ownership.