A mistress is a woman who is in a relatively long-term sexual and romantic relationship with a man who is married to a different woman.

Is it against the law to sleep with a married man?

Several state legislatures statutorily prohibit adultery as a crime. Under some statutes, both parties to an adulterous relationship are guilty of a crime if either of them is married to someone else. Other statutes provide that the act is criminal only if the woman is married.

What is it called when you are married and have a girlfriend?

Polyamory (from Greek πολύ poly, many, and Latin amor, love) is the practice of, or desire for, intimate relationships with more than one partner, with the informed consent of all partners involved.