I find sex on acid is great, but hard to reach a finish, but it is constantly plesurable and so energetic that the lack of an end is not an issue at all.
Sex on mdma... well I am one of those who find it hard, although I find making out and cuddling to be alot of fun.
Sex on 2c-b/i insane, I hardly knew what I was doing, it was like crazy instrincts, crazy monkey sex. Larger slightly delerius dose for me though, interested to see what happens on a smaller dose.
Once having sex on acid my girlfriend apeared dead with no eyes... I was disturbed for a few secconds but a bit of strong headedness and love made it not very hard to fix her image in my mind :P
Also once my girlfriend punched me in the face by mistake, which ripped my septum peircing slightly and caused me to bleed alot, she was quite in shock but we managed to calm her down and ended up enjoying the red addition to the night, quite sticky!