This does not mean that if you have lusted in your heart, that you should just go ahead and act it out. It would be bad enough to sin with the mind, but much worse to commit the act. Whether you have committed this sin in thought or in deed, you need to know that no sin is too great that He will not forgive. Also, no sin is so too small that we don't need to confess it to God. Another perversion of God's design for sexuality is the sin of fornication.
Fornication involves any sexual relations between non- married people.
The single person who gets involved physically with someone of the opposite sex is breaking the 7th commandment, because there is something missing in their relationship - commitment. If you really love each other then get married, and wait until that wedding day. Many couples break this commandment during their engagement by fooling around, only to later experience guilt and regret. Sex is God's gift for married couples, and it is wrong to open that present beforehand. Some are like the child who just couldn't wait for Christmas day to arrive so he snooped around and opened some presents early to find out what he was getting, only to experience a huge let down on Christmas day.