Dolphins don't rape humans.
Raping a human requires the dolphin to be able to physically subdue a human and taking off the human's clothes the lack of opposable thumbs and hands really make it hard.
They can, however, "hump" a human and conduct in a way that if was made by another human being would constitute sexual harassment and molestation. Here are two videos as an example:
A dolphin with an erection shoving its snout to a woman's crouch:
http://www.youtube.com/watch?v=LvpJdNiqZJc
A dolphin dry humping a visitor in a water park:
http://www.youtube.com/watch?v=qGB8_bWkOA8
All the claims about dolphins raping humans, seem to be from this source which includes as its sources this picture:

which is obviously a very bad Photoshop. They also mention Scott Randleston of the Dolphin Research Institute of Boca Raton as a source, the website for the institute doesn't mention Scott Randleston, and searching for his name just brings up this claim repeating.
This behavior is not limited only to humans, or even living creatures, as mentioned In this article by Jean-Pierre Sylvestre:
It's noted in Pilleri et al. (1980) when 3 toninas were kept at Duisburg Zoo in 1978 that the adult male 'Vater' once tried to rub its erect penis against a large brush lying on the tank floor.