Wow, although I am the husband I still know my wife pretty well and can say no, she will not have sex with a guy just to have sex with him. Mrs. WS has to have some kind of chemistry with him and really be turned-on by him to get naked with him.
I will say though that chemistry goes pretty far. She has not been attracted to some men I thought for sure was her type (and physically they were but they just didn't click) and she has been all over other men I would have never thought in a million years she would be attracted to.
So although she would never go for the "mercy fuck", she has surprised me over and over when I try to second-guess her on who turns her on.