You can also play doubles pinball with two players at once, one on each flipper. This works best if the team involved likes each other enough to cooperate at juking the machine around. (As for the pinball and sex hypothesis implications, my boyfriend and I sometimes play doubles pinball with our arms around each other's waists to better follow each other's movements, but alternatively the teammates can agree beforehand who'll be the dominant partner... I mean, in charge of shaking the machine. I digress.)
Multiplayer games (by which I mean those with more than two players involved) are much more fun when played in this style; my friends and generally I play rotating strictly in one direction, tagging in and/or out only when a new ball comes into play, but on more daring days we switch at every ramp and ball lock, or even every given opportunity. If more than three people are playing, the situation is likely to degenerate into full contact pinball... not that there's anything wrong with that.