You can file for a divorce if your spouse has cheated on you, i.e. when they have had voluntary sexual intercourse with another person. This is also referred to as adultery.
To get a divorce, you will have to prove that voluntary sexual intercourse took place between your spouse and another person.
Till recently, cheating was a crime. It is no longer a crime. However, you can still file for divorce on the grounds that your spouse has cheated on you.