Umm, it's a real thing that really happens.

PS: http://xkcd.com/386/ Duty Calls, but I am at work and can't post specifics... argh first world problems indeed.

Explanation: http://en.wikipedia.org/wiki/Pegging_(sexual_practice)

This is one of the worst trolls i've seen on HN. Obvious ignorance of pegging, flame-baiting the downvoters and trying to link the possibility of alternative sex acts with physics.

She could bang him with her fist too. That would be from behind, and completely feasible with human anatomy. Clearly you haven't been on the internet very long. I can probably find you video evidence in about 20 minutes...