Some situations could force some people to sleep together just like some unfortunate event could happen. They make not have bad intentions. Though some may sleep together because of the desires of the flesh, some may be forced to do that not out bad intentions.
The Bible condemn the sexual act between a man and a woman who are not legally married. Even Joseph and Mary in Bible didn't have a sexual relationship when they were not legally married.
A man and woman can have a sexual relationship if they legally married.
1 Corinthians 7:1-2 Now concerning the things whereof ye wrote unto me: It is good for a man not to touch a woman. Nevertheless, to avoid fornication, let every man have his own wife, and let every woman have her own husband.
To have a sexual relationship with someone without being married to the person is prohibited by God. You have to be married to the person before you touch that person.