It's not an unreasonable question.
But the answers are pretty simple.
Can a man be raped? Only by another man, because the definition of rape requires a penis.
Can a man be sexually assaulted by a woman? Yes.
Is that just as bad as rape? It can be, the same way a woman can be sexually assaulted in a way that doesn't involve a penis and it can be just as bad.
Does it happen anywhere near as much as men assaulting/raping women? No. But that doesn't mean it's not serious when it does happen. One person or group's suffering does not diminish or take away from that of another group. There's not a limited amount of "we should stop sexual assault" to go around.
Do I need a signed bit of paper every time I have sex? No, you only have to reasonably believe that your partner consents.
Is it hard to tell if my partner is consenting? Not really, not if you are a normal reasonably human being.
Is an erection a guarantee of consent? No.
If I did X in the past, does that make me a rapist? Not if your partner wanted X.
What if they didn't really want to, but went along with it anyway? If they agreed because they like you and want to make you happy - probably not. If they agreed because they were scared of the consequences of not agreeing, it might be rape. Consent must be freely given.
Have I missed anything?