One sided, clinical and awkward consensual sex. Typically used to describe sexual activities in a relationship wherein one party is with the other out of pity and/or because one (or both) of them simply wanted to be in one instead of building a future with their partner. Often an indicator that such a relationship is not long for this world.
"Bro, my roommate Dan was getting the Wilmslow Express in our dorm. Emily deserves better than that creep."
"Yeah, I wouldn't be surprised if that relationship is over by winter break."