Now, I dont' really have an explanation, but I think it's the same thing (relatively) as people getting erections on MDMA. I can't do it. I can get a "half-stack" but that's about it. Nothing I could really do with it. Not that I'd want to anyway, but that's entirely another story. But many people, even people here on Bluelight will praise (or condone) sex on MDMA. Could be a lot of different things. First of all, I'm sure a lot of it is mental. I'm sure if I had to, life-threatening situation or whatnot, I could get an erection on MDMA too. But why do that when I can have a girl comb my hair with a brush? It feels much better in my opinion. Also, some of it also probably has to do with the mechanisms in the brain. Perhaps it has to do with the fight-or-flight response. Kinda like when I ran cross-country. Always get the worst shrinkage when I run, because frankly, the blood has much better things to be doing in my body.
I know I didn't really answer your question, just some thoughts.