I think WE, HERE at PD can reason away why jokes about taboo subjects can be "ok" or "not ok"...but I question the level of meta-thinking your average Joe will indulge in over the matter. I think most humor is learned, just like I think most bigotry against folks is learned, and I think most folks KNOW that rape is wrong without needing to know anyone who went through it.
Male on male rape is one of those ills of Western civ you just don't hear about to the degree it lives with you the way male on female does, just like incest is known, but the degree to which is happens would probably shock most folks who believe it's a rare entity. It's not. "Prison rape" gets the treatment it does because folks tend to not see an inmate as a victim. They "deserve what they get" because they are incarcerated. Every evil that visits them is what they get in return for their misdeeds, regardless of circumstance.
So to humanize the inmate would mean that we treat that person as we would ourselves--wanting the best possible for them. Society demonizes inmates and treats them like they don't deserve anything but badwrong anything, badwrong medical practices, badwrong food, badwrong conditions. Why not add rape into the mix and then laugh HA HA buttrape is funny HA HA?
I think throughout history, you'll see that dehumanizing the victim in a heinous act will actually allow those of us who would otherwise change that act or make it less acceptable to hold off and hold back. That's not to say we sanction it, but neither do we actively participate in its demise, either.
I know I'm saying something a little bold, here, but I can't remember how many times I've heard "I hope you meet 'Bubba' in jail and get what you deserve"...what does that say? That we HOPE that people get their asses torn up. As punishment. I'm sure it's said in jest, I'm sure those saying it don't actually literally hope that, but it would certainly be a different proposition altogether if someone said to a woman, "I hope you're met by a gang of rapists in the dark of night and ridden till the cows come home."