Of course it's possible to rape a man. Just because he becomes erect doesn't mean he desires sexual intercourse. People are more than the instictive reactions their body produces. Someone who chooses not to engage in sex, but is coerced into doing so, is raped, regardless of his body's physical reactions. The key element is coercion. Erections or other bodily responses have nothing to do with it.
Consider a murderer who ties his victim to a chair and rigs a diabolical device. A hammer strikes the victim's tendon below his knee, causing his leg to reflexively kick and set off a bomb. The murdere is caught but argues that the person committed suicide, since he kicked the bomb switch himself.
There's another spin to this too. Since men apparently aren't raped if they get an erection, then it follows that women aren't raped if they lubricate. You could take it one step farther, and say that neither sex can claim rape if the victim has an orgasm or if they get aroused in *any way*. Courts never cease to amaze me.