i take it as a scene from "y tu mama tambien" (a great movie. go check it out.) in which the two best friends realized they both cheated on their girlfriends with the other's. not the same situation, but similar in the sense the two friends described eachother as "cream brothers".
That AO faitful is why i have the same "ehhhh" feeling. Even if my friend is ok with it, i'd still know that remnants of him are everywhere with this girl, and even so, if this was not the casde i still wouldnt want to imagine my friend doing anything sexual or intimate while i;m doing that. it's just disturbing.