My friend stumped me with this one:
When a married man has a woman on the side, she’s referred to as his mistress. What is (if there is at all) the gender specific term for a man having an affair with a married woman?
We tossed out everything from “lucky” to “F*ck on the side” to “gigolo” but weren’t satisfied with them.
So…any suggestions?
“Kept Man”
“Personal trainer”
“Boy Toy”
How 'bout boy toy?
How 'bout “Personal Stud”? 
or
Recreational Gentleman
Personal Escort
…it has never been my way to bother much about things which you can’t cure.
Yeah, boy toy certainly works as a slang/vernacular term (as do pool boy and gardener, nyuck, nyuck) but, it seems to me that mistress is pretty much a formal term and that there should be some corresponding term for a male lover out-of-wedlock.
Well now, wait a moment. A “mistress” as I understand the term is a woman supported by a married man in return for love, understanding, and (ahem) sexual gratification outside of his marriage.
But your question alludes to a man who is simply “having an affair with” a married woman. These are two quite different scenarios. The first of my examples applies only to the male equivalent of “mistress.” The other two cross the spectrum.
Nickrz:
Well, o.k. then the formal male counterpart to a mistress (by the above definition) would be a gigolo which the crummy Merriam Webster Dictionary here at work defines as “a man supported by a woman for the purposes of blah, blah, blah”.
So, this muddies up the water a bit. What then are the formal terms for lovers outside of marriage (other than adulterer and adulteress) who are not supported by his or her paramour?
“Lover(s)”
El Mariachi Loco is a translator, and if he sees this, he might help.
In Mexico, we call a male “mistress”: Sancho, and Gonzalez.
The term “boy toy” refers to a woman who has many male LOVERS. When Madonna was young and wore all the rosaries and crosses, she had a belt that said “boy toy”, referring to herself. I think when speaking of a man, the term would be “toy boy”.
“Man prefers to believe what he prefers to be true” -Albert Einstein
Women of nobility referred to the male counterpart to a mistress as a “favorite”.
I vote with Nickrz; I think the term is simply “lovers.”
-Melin
Phenomenal woman
Bitch Corporate Lawyer
That's meI think the official term is “paramour” but I don’t like it as much as mistress because it’s easy to misinterpret. I mean it’s not as well known that it refers to a married person’s lover.
Lucky bastard?
Cogito Ergo Vroom
I think therefore I ride fast…
Consort.
My aunt Mila has one. He’s twenty-seven years younger than she is, and right now they’re living in Prague. He’s a Greek shaygets named Stavros. We all call him Aunt Mila’s consort, except my mother’s mother, who just calls him “the shaygets.”
–Rowan
Shopping is still cheaper than therapy. --my Aunt Franny
According to Webster’s, “consort” means an associate or spouse (though that’s not how I always understood the word); but “paramour” is defined as “an illicit lover”, which sounds pretty close to what I think you mean.
I knew, I knew, that reading tawdry historical romances would come in handy some day!
From Merriam-Webster
Main Entry: ci·cis·beo
Pronunciation: “chE-ch&z-'bA-(”)O
Function: noun
Inflected Form(s): plural ci·cis·bei /-'bA-"E/
Etymology: Italian
Date: 1718
: LOVER, GALLANT
Note: in all the romances I read, this word is used for a woman’s paramour, when “mistress” would be used for a man’s paramour.
HOW do you pronounce it? I couldn’t make heads or tails of that. Great word though, I’m thinking of getting one myself and that will come in very helpful. 
Hm. I guess “mister” just won’t do.
How about “side boy”?
“My husband’s out, so I’m inviting my side boy over.”
Your Quadell