Is it illegal to sleep with someone else while married?

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

Is it illegal to date a married man?

It is illegal From a legal perspective, having a relationship with a married man or woman is illegal. While there may be no legal case against you, he will stand to lose his marriage, his property, and may even have to pay financial assistance depending on how his wife chooses to proceed with the case.