Mark + Sarah as a Couple?
Sarah tells Mark she is "looking for something different" when they first kiss. We assume this means a long-term commitment and not just casual sex. Then, later, we see what Sarah means by "different", that she has her first orgasm from S&M.
So, Mark knows that in order for Sarah to be happy, he would have to get into some kind of S&M, or D/s thing with him being the D and her being the s. I guess he would be... spanking her?. The girl seemed to be hard core. Could she be happy with a belt cracking across her ass instead of the lash being used on her back?