Men can be raped in various ways, as long as it's non consensual, it's rape.
One of the notorious cases is a guy at a party passed out drunk, and she took advantage in his sleep. He can't say no and his body reacted naturally. Male arousal is not a defence argument of "He clearly wanted it!" against rape. That's up there with "the body has means to shut itself down" in the history of utterly stupid things said.