Have quite few experiences with MDMA and sex. One really hot experience involved me and a female friend, after everyone else at our place was coming down. The room was pitch dark and we were talking about the things we'd watched during the night and how the group (about 8 people) were all comfortable and allowed themselves to "be themselves in the environment". That seemed the beginning of things, although she and I were not terribly attracted to each other but open sharing conversation was certainly a turn on. We were lying on floor cushions and those accidental bumps of legs and arms while disclosing thoughts and fantasies caused us both to think out loud..."Is it ok if I touch you...here?" "Sure, can I touch you?' "Of course...". We established that we weren't trying to totally have intercourse as she hadn't broached that with her partner in the moment (my wife was ok with things), which was fine. We both were totally reveling in the intensity of not being able to see each other due to the darkness, just hearing each other's voices, breathing, and the heightened sensations of touching.
With one hand she touched herself and with the free hand touched me, needing skin, and I did the same. As things escalated we somehow continued talking, describing what we were both doing and feeling as we did it. The sounds we both were making as we stroked ourselves slowly were just echoing in our ears, we stretched legs across one another for as much skin contact as possible without actual genital penetration. This got more and more intense where we wound up in a kind of a position that allowed us both to be occasionally able reach out and finger/stroke the other for a while. She got wetter, I got harder. She started to come, which was beautiful to hear and imagine seeing, then she just held my balls and I came really, really hard. That was wonderfully insane, and it completely took our breath away for awhile.
*This was one of my first experiences and it's when I learned that struggling to get an erection while on E is certainly something that doesn't need stressing over. There's so much erotic that can happen in time.