manstress

noun

1. (slang)A male equivalent of "mistress"; a male object of one's affections who lies outside of one's primary relationship.

   “Tom and Nicole had been married for several years, but it was well known that she was keeping a manstress on the side.”

   “4. Thou shalt commit adultery, or preadultery: If it feeleth good, do it. Particularly if you're French, and keeping a mistress or manstress is expected.”