I think everyone has an understanding of what to “know” in the Biblical sense means. In case you don’t, it means having sex. I didn’t understand why as a kid—I remember hearing, “He ‘knows’ her…in the Biblical sense” and (being a small genius) figuring out what it meant from the context of what the adults were saying. But, I wondered why the word “know” had anything to do with it.
So, now that I’m married and somewhat grown up, it makes sense to me. Sex (ideally) is the full physical expression of a deep, intimate relationship where two people perfectly and unbreakably committed to one another through the bond of marriage share everything they have—their whole selves. One of the major images of God’s relationship with His people in the Hebrew Scriptures is that of a marrital relationship, in part because of the intimate knowledge that God wants to share with us. It carries over to our Christian tradition in our calling the Church “the bride of Christ.”