I knew many single women at the law firms I worked at who had not-so-secret, secret affairs with married senior attorneys or partners. They probably did it for a number of reasons: career advancement being one, and the fact they didn't have other options being another.
I'm sure there are more. :-)
We've either worked at the same firms, or the situation is rampant. Sadly, I think it's the latter.
And, many of the married partners were married to their former mistresses, now trolling for number 3 . . .