1 Thessalonians 4:5-6

5People who do not know God strongly want to have sex in a wrong way. They cannot control themselves. You must not be like those people.

6No man among you should ever have sex with the wife of a Christian friend. That would be the same as if you robbed him. It would be a bad thing to do against him. Remember that the Lord will punish everyone who does things like that. We have already told you that. We warned you very seriously about it.