Word for a "Male Mistress"

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)

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.

That would definitely be slang (slang that ignores the fact that words ending in -ess usually denote something feminine, for that matter). "Paramour" is more unisex and means something similar, though.

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.