"Nature" is nothing; God designed us. He programmed the nature of sex. He intends us to use His gifts according to His design. Regarding the "silly words", it's called making a covenant of marriage. If you think that's silly, take it up with God.
We don't need to. The Bible does that for us, and yes, it can, because it is God's word, and He can. Yes, it is a blanket statement. It's sin. Don't do it, because it will do damage to your inner being. It exposes you to disease and unwanted pregnancy. All sex outside of marriage is a corrupting influence. God loves us as His kids and does not want His kids corrupted by illicit sex. It's pretty simple.