How Does the Bible Define Sexual Immorality?

"Let marriage be be held in honor among all, and let the marriage bed be undefiled, for God will judge the sexually immoral and adulterous" -Hebrews 13:4. 

When warning the wicked of the consequences of sexual immorality, we often get a response of, "What's immoral about what I am doing? Everyone else is doing it! The Bible doesn't even define 'sexual immorality.'" Who are you, we are asked, to judge what I do? 

First, let us ask what consequences the Bible warns us will apply to sexual immorality. Is it important enough to worry about it?

Here is the Bible's answer: