There should be no artificial boundaries placed on what we can and cannot explore in art. To say that "you can go here but no farther" begs the artist to go not only there but beyond. There may be a time in which something is not appropriate - painting someone falling from an office building on September 12, 2001 would have been in poor taste. In 2007, we can include an image of someone falling from a building and it doesn't raise any greater eyebrows or indignation than it would have on September 10, 2001.
We should be able to show images of human sexuality, in all its flavors, in an artistic medium. To back up a bit and address an earlier comment, I should distinguish something - the act of photographing a nude human figure, male or female, need not be any more an erotic act than photographing rocks and trees. Frankly, shooting nudes is a lot of work! I don't have time to get horny when working with a model. I won't claim that I don't want my viewers to be erotically inspired by my work, but I don't want it to be their ONLY response either. To me that is the distinction between art and porn - porn provokes a single response (in a willing consumer) but art provokes multiple responses in the same viewer, often at the same time. Porn exists inside a container (magazine, DVD, etc), which can be put away at will. Art exists in a public space where it continuously interacts with its audience.