This is a topic that you will get some differing opinions on. There are old fogeys like me who believe that sex almost always forms or breaks a marriage, that if a man has sex with a woman he has married her or committed adultery with her. This can get very heated and no one is going to hijack this post to have the debate again because normally I'm the one who does that and I've resolved not to this time. The point is that when you start down this rabbit hole of defining marriage from a biblical perspective it changes everything, all of your assumptions have to get left at the door because it doesn't just add poly into the mix, it changes monogamy and celibacy and especially divorce. And none of these things requires government involvement in God's eyes so you really have to start pulling on some strings to unravel all this stuff.