It just so happens that there *actually* are known medical conditions that makes it so that some people, uh, feel less, generally speaking. Now, imagine mixing that with someone who would either naturally have high stamina, or already has healthy life habits - you can imagine that some, er, sessions, could last a hot minute.
Now, obviously this is a fantasy setting, but at the very least there could be some actual truth to the story being depicted here, even though there's a healthy amount of creative license in play.