The following question came to me via Formspring:
Is there anywhere in the Bible that specifically lists what sexual impurities refer to, or how far "too far" actually is?
Here's my answer:
The short answer: no, not specifically. This is related to something we were talking about last night at my church, actually. There are two concepts in the Bible that you have to understand: "Law" and "Grace." "Law" means a set of rules that you follow in order to keep things square with God. The Ten Commandments are the most famous example of this, but the concept is throughout the Bible. "Grace" means that you are square with God because of the sacrifice of Christ Jesus on the cross. Under Grace, whether or not you follow rules and regulations does not determine if you are right with God; whether or not you accept Jesus' sacrifice for you determines whether you are right or wrong with God.
That is not to say that there are not right and wrong things to do even if you are living under grace. For example, a Christian under grace is not allowed to rob a convenience store at gunpoint. If he does, although he is forgiven by God when he confesses his sin, he is not forgiven by the owner of the convenience store or by the local police department. There are consequences of his actions which are not wiped out by Grace. So, there are "right" things to do, and there are "wrong" things to do, even if they are not necessarily always codified in specific sets of rules in the Bible.
The reason the Bible doesn't necessarily set up specific sets of rules for every circumstance is that, first of all, God doesn't WANT us to live under a Law. Part of the reason God gave the Law to the nation of Israel was to demonstrate how impossible it is to always perfectly follow a set of rules. Living that way is a losing game. And second of all, God has actually written His Law on our hearts (see Hebrews 10:16 http://esv.to/Heb10:16). If you have accepted God's sacrifice through Jesus, your heart knows right from wrong. You know when you've crossed the line; you don't need a playbook with diagrams and a tape measure to know whether you've gone "too far." Under grace, The Law which is external becomes unnecessary, because God's Law is internal to each of us.
But you don't need a rulebook to determine whether you have sinned or not anyway, because from God's perspective, the sin is not something you do with your body, but something you do with your heart. Matthew 5:27 (http://esv.to/Mat5:27) says that if you even look at someone with the intention of having sex outside of marriage, you have already sinned, whether you actually follow through with it or not. This is not to say that you can't be attracted to someone and desire a physical relationship, because sexual attraction was created by God. But the instant of the sin is probably sometime around when you go from "I would really like to go to bed with that person" to "If given a chance, I WILL go to bed with that person."
So: the Bible doesn't particularly give specific "how far is too far" rules, although doubtless you could find some in there if you look around enough. The reason why is that you already know the limits, because God has written them on your heart; if you choose to look for rules and regulations, you don't have to look any further than there. When the sin has occurred internally, the sin has occurred from God's perspective. My advice to you is: keep your mind pure. Keep your hands from going where they don't need to go. Keep the parts of your body that need to be covered, covered. And if you make a mistake in your heart, bring it before God in prayer and make it right; don't beat yourself up about it. Live under Grace; don't go looking for the Law to tell you what to do, because the Law is merciless in meting out punishment when you mess up.