Adulterer (or adulteress) can be used for anyone who has a sexual relationship with a person who is married to another, but again does not always imply the specific social situational that mistress does.

Is it legal to sleep with a married woman?

There is nothing illegal per se about having sex with a married women.

When you are separated is it cheating?

Couples who are separated, whether informally or legally, are still married in the eyes of the law, regardless of how independent their lives have become. This means that if either spouse has a sexual relationship with another person during the separation period, they have probably committed adultery.