Is talking about or discussing sexual body parts and genitals with a boyfriend or girlfriend sinful? I'm wondering if just talking about it is considered sinful, not touching each other or showing them the areas.
"But among you there must not be even a hint of sexual immorality, or of any kind of impurity, or of greed, because these are improper for God's holy people. Nor should there be obscenity, foolish talk or coarse joking, which are out of place, but rather thanksgiving. For of this you can be sure: No immoral, impure or greedy person -- such a man is an idolater -- has any inheritance in the kingdom of Christ and of God. Let no one deceive you with empty words, for because of such things God's wrath comes on those who are disobedient. Therefore do not be partners with them" (Ephesians 5:3-7 NIV).
We can generalize this rule to basically say that since it is wrong to have sex outside of marriage, or look at pornography, or to lust after someone's wife, Paul is saying that it is equally wrong to say things that makes it sound like you approve of these sins. Often the things you are talking about can easily slip over into dirty talk, which Christians are not to participate in.
Talking about about your genitals with someone of the opposite sex will leave the impression that you are hinting that you are wanting to have sex. Boys and girls typically get involved in this because its arousing, in part because you know your parents would not approve of it and it triggers the imagination. "Stolen water is sweet; and bread eaten in secret is pleasant" (Proverbs 9:17). We can forget that the thrill of doing something you know you should not do can lead to arousal.