Is the male version of a mistress, a mastress? It's a term I would use, but I don't know if it is just slang or if it is formal...
P.S. I mean a male that sleeps with a married woman (love, not business)
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Personally, I would call such an unofficial sexual partner a lover, no matter what the genders of the people involved. The online Merriam-Webster's dictionary's definition of lover includes the following meaning:
someone with whom a married person is having a love affair
So, while the word does not necessarily imply a man sleeping with a married woman, it most certainly can be used that way.
If you're looking for a disparaging term, you could use boy toy, defined by dictionary.com as:
Also, toy boy. a young man noted for his good looks and sexual prowess, especially one who maintains relationships with older, more powerful persons.
As J.A. said, "paramour" is a gender-neutral equivalent, and a good way to express a "male mistress", so long as there is a some gender-context provided for the reader/listener. But the question specifically asked for a male-equivalent, not a male-acceptable equivalent.
Technically, Cicisbeo is the word you are looking for. However, I had not even heard of the term before today. It could be a product of merely ignorance, or its lack of prevalence in the USA. I won't speculate.