There are a few misconceptions in your logic.

1. Force is required to rape
2. Erections are controllable

Both of them are easy to disprove, but not obvious at first sight.

For 1 consider any case where a woman might have power (not physical) over a man, e.g. blackmail, teacher, parole officer, boss, etc. Another possibility to remember are weapons or physical threats to a third party. Also you should remember that humans have a fight/flight/freeze response, so a third of humans would just freeze regardless of being able to overpower their attacker. Finally there's also the possibility of even without any threat, even being able to think properly, and knowing that he could physically overpower a female attacker, a man might not do it for fear of legal or moral repercussions, e.g. being thought not to hit girls or believing that no one would believe that he was defending himself. In fact lots of women who get raped don't try to fight back or escape, believing (sometimes accurately) that their attacker would worsen the offense if they did that, e.g. by killing them (even if no threat was made), it's not uncommon for rape victims to feel ashamed and guilt about not having fought back, and by saying that men can't get raped because they could theoretically overpower their attacker you're indirectly saying that any woman who doesn't fight back with all her might is not being raped either, because they could have overpowered their attacker of they tried.

For 2, erections (and even ejaculation) are physical responses, in fact you can make a corpse get a hard on and cum (some wives do it to preserve their husbands sperm). This is no different from women getting wet or having orgasms while being raped (both of which are common), it means nothing, it's just a physical reaction to a physical stimulus. In fact lots of victims (both men and women), especially those in abusive relationships think they deserve that because of those physiological reactions. To put it in simpler terms, saying a men can't be raped because if they got an erection it means they wanted it is like saying that people can't be stabbed because if they bled is because they wanted the knife.