1 Corinthians 6:16
What? know ye not that he which is joined to an harlot is one body? for two, saith he, shall be one flesh.
Note 12 at 1Co 6:16: This is an amazing statement that very few people have thought through. Individuals don't have to enter into legal marriages with others to become one flesh with them; any adulterous relationship produces this one-flesh union (see note 4 at Mt 19:5). An ignorance of this has led many people into illicit sex, thinking that when it's over, it's over. That is not the case.