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.
In a video that went viral on Monday, minister Bisahulal Singh, who is contesting the 3 November Assembly bypoll from Anuppur, was purportedly heard describing the second wife of Congress candidate Vishwanath Singh Kunjam as "mistress".
"Why Vishwanath Singh is hiding information about his first wife and mentioned his 'Rakhail' (mistress) in the nomination papers? He has not given information about his first wife, but about his mistress," he was heard as saying.
Yes I have read many definition for Mistress, and yes it is offensive word...I show this word used for those woman who are prostitutes many many times even though it mean as I said in my post or your definition...Gauri's marriage is illegal, but Jagya gave her all right of wife, He introduce her as his wife in society, He also fight with his family for her. In this case she is not mistress...yes legally Anandi is his wife but he himself didn't give the right of wife to Anandi.
According to the media reports, Salman Khan and Sonam Kapoor starrer Prem Ratan Dhan Payo had to cut three scenes for not being suitable for viewing. A dialogue in which word 'rakhail' (mistress) was used, a Ram-Leela sequence and a man getting hanged were chopped off from the movie.