Before I changed majors I was a criminal justice major and took a lot of law and law enforcement courses. The teacher was a retired cop, he said though it is a hard case to prove it does happen and that there may be more cases that are unreported for the same reason a woman won't report a rape...embarrassment.
Perhaps even more so that and the fact that most people don’t believe the man, because it is hard to understand for most people that the body responds in ways that we can’t help.
I think there was a Law and Order SVU episode on the topic too.