All men are rapists

Lately I've become interested in debates between feminists and men's rights activists.  In particular, there is an increasing amount of pressure by feminists to address "epidemic" levels of domestic violence and rape.  Much of it borders on hate speech.  There is one aspect of this I would like to take issue with: the idea that all men are capable of rape.  The story, at least as told by feminists, is that we are living in a "rape culture" and that men must be conditioned not to rape.  Some feminists go so far as to assume that all men are rapists until they can (somehow) prove otherwise.  By that logic I should assume that all women are liars.  Statistically I'd have a much better chance of being right.  Or perhaps assume that the black teenagers hanging around the street corner are potential muggers.

My question is simple: are all men even capable of rape?  And by capable I mean actually physically capable.  Can they achieve an erection under circumstances akin to rape?  Human males are quite different from just about any other mammal in that they have no bone inside their penis.  An erection is maintained by blood-flow alone.  This also means that they cannot simply have sex on demand.  Something must arouse them.

I hear a lot of talk about how men must impress women in order to gain access to them--by becoming an "alpha" or whatever.  But the implication of male physiology is that the female must also impress the male by sexually stimulating him.  It's an entrenched position that men rape far more frequently than women, at least when it's even acknowledged that women rape at all.  The typical argument against women raping men is that it is impossible to force a man to have an erection.  This same argument can also be used to show that not all men can rape.  Just as they cannot be aroused by being forced into submission, they are not necessarily aroused by forcing another.

I find the notion that "all men are potential rapists" offensive in the extreme because it implies that all men are violent psychopaths who are sexually aroused not only by violence, but by their own violence.