Genesis 2:24

Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh.

1. Grown men should not remain at home with their parents, but should marry, love their wives with all their hearts, and become as one - that is, totally united.
2. A married couple should have a sexual relationship.
3. Adam was speaking in the previous verse. It's not clear whether he still is here or not. If he is, then he clearly understands the concept of mother and father. If not, then this conclusion has been drawn by someone else.