Thanks for the detailed explanations. Based on that, I think it is possible the way I've written it. And like Prometheus brought up, it can be implied that the victim was still somewhat alive.
A friend also mentioned the idea of having the protagonist (murderer) stimulate the penis before the man is shot. In the scene, they are already embracing and kissing at the exact moment she shoots him, so it wouldn't change much. However, it's hard to know what amount of stimulation would be required to induce a full erection. Even though it is a primarily physiological process, the man would be in a fight/flight response so sex would be quite a low priority to his brain.
So I think my story is good to go. Thanks again for all the help—the information provided has been quite invaluable. For those concerned about my interest in such a grotesque scenario, I'm writing a collection of short stories and this one is about a school shooting. I really ran away with the ideas, though.