manstress

Etymology
, not attested before 1980.

Noun

 * 1)  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.
 * 1) * 1997, Edward P Moser, Words to Live By, published in the Wall Street Journal on the 25th of March, 1997, as quoted by "Dreyfus2u" (username) on the 26th of March, 1997, "Need WSJ article 3-25-97 -- RE: 10 Commandments...", in, Usenet:
 * 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.