This week’s phrasal verb is “cheat on”, and it is used when we want to talk about a person having a romantic or sexual relationship with another person who is not their romantic partner. For example:
Bill is going to divorce his wife because she was cheating on him.
I think my husband is cheating on me. Last night, he came home really late, and there was lipstick on his shirt collar.
If my boyfriend ever cheated on me, I would break up with him right away.
Peter’s girlfriend would never cheat on him. She loves him so much.
So, with this expression, it doesn’t matter if the person is married or just dating, but they must be in a romantic relationship.
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