It is possible...it's not a question of whether he is aroused or not, but one of consent. Even though he is aroused and she hops on for a ride, he may not want it even though his body is reacting to the situation, stimulii, etc that she is providing. Up to about fifteen years ago the argument was always in court that if he had an erection he must have wanted it, but that has since changed. Then you have the example that binary brought up in relation to the guy actually being penetrated. To break it down, legally, the term rape in legislation was replaced with sexual assault...that being basically an assault, or force exerted against someone else and not of their consent, of a sexual nature, but involving penetration, which can include, vaginal, anal, oral...hope this helps
