French = Whenever France is invaded or theatened, their troops drop to their knees and break out into spontaneous blowjobs on the invaders.  It's a well recognized medical condition that has yet been explained.

Greek = The Greeks have historically been known to indiscriminately stick their dicks into anything and everything, even the out door.

Russian = It's just too damned cold to get naked, so the guy just pulls down his fly and the girl just opens her shirt and then they do what comes natural.