If the woman that a married man is having an affair with is called his mistress...what is a man called?

Would the man a married woman is having an affair with be called her master?

Obviously I ask strictly for edumication purposes.
Update: I asked my husband this the other night and he said...he would be called "a dead man." I giggled nervously and then texted Poopy to tell him he better be packing.
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