You sound like the only person making assumptions here.

I'm not saying that men can't be raped (though, in many states, legally speaking, they cannot). Nor am I saying that men's sexual assault is less serious than women's sexual assault.

I am saying, however, that men are much less likely to be victims of rape, or any other kind of sexual assault over their life course, than women. Therefore, when talking about rape, the de facto meaning is of a man raping a woman.