Hi, Rachel . . . I see this is still going. I am Bill . . . good to see you

You asked, what is marriage? I'll offer a little of what I have >
Well, Mary and Joseph were engaged; and if Joseph was surprised that Mary got pregnant, then it would seem he had not been having sex with her so she could get pregnant. And they were a godly couple, I would say, who would do things God's way. If sex would have been ok, why wouldn't they?
"'If a young woman who is a virgin is betrothed to a husband, and a man find her in the city and lies with her, then you shall bring them both out to the gate of that city, and you shall stone them to death with stones, the young woman because she did not cry out in the city, and the man because he humbled his neighbor's wife; so you shall put away the evil from among you.'" (Deuteronomy 22:23-24)
So, it is a death penalty offense to have sex with a virgin who is another man's fiancée. So, cheating while engaged is a death penalty offense. And it looks like while the woman is engaged, she is a virgin, though she is called her fiancé's "wife". If sex would be ok, why is she still a virgin? Well, it could be she is too young but has been promised, "but it doesn't say anything about the age of the virgin!"
I would say that marriage is when two engaged people end their virginity with each other. However . . . Paul in 1 Corinthians chapter seven was writing to Christians who would be family all together. So, couples who belonged together would be known to the others, and they would understand who was with who. And I would say ones would not go on to sex without first having agreement with their other Jesus family people. There would be accountability, not secret stuff. And the real leaders are approved by God; so if God trusts our really Christian leaders, He certainly expects us to trust them, too, including by obeying what they say about when sex starts > Hebrews 13:17. So, I would say marriage begins after there is oneness with trust with our other Jesus people whom we know God trusts, and when then we have sex.
"Because sex for Christians is not just a physical act, but in trust and intimacy with all our other Jesus family people. It is an act of love, which involves trust and honesty."