I stayed over in friend's flatmates room and found some really tangy stains?

I am truly sorry that you had to sleep in that bed. That's absolutely disgusting. As a guy I have no problem with the fact that this guy is sleeping with girls (because really what guy doesn't want that?). However, if I were him I'd be sure to change the sheets before a guest stayed in my room. Really it's only a matter of washing the stained ones and putting out fresh sheets. It was funny to your friend because it wasn't her having to sleep there, but otherwise this is not funny at all....just revolting.