LSD and 2C-B are the best drugs for enhancing sex for my brain subjectively, but adding MDA, MDMA or 6-APB to either of the two has also proven to be incredibly fruitful. I've never had problems with E.D or delayed/impossible climax when on these drugs like some people do, but I also abused amphetamine from the ages of 18-21 like my life depended on it. Nowadays I'm kind of hardheaded towards amphetamines at large, I've never experienced a comedown or any negative aftereffects from even 600mg of MDMA/MDA, I just feel like I'm radiating this golden light for days after. I'm saying all of this to paint the context through which I interact with these drugs, and my subjective relationship to them.
To answer your question, I'd say that both the aforementioned psychedelics (LSD/2C-B) and the empathogens (MDA/MDMA/6-APB) have a stimulating effect, they enhance tactile feelings intensely, but the reason that they really excel for sex is that they make the feelings of love so much more intense. I would have no interest in banging down some stranger while I'm rolling and/or tripping, but my partner right now is the love of my life and he's my best friend. Those feelings of the connection shared are much more evident and in your face when rolling and/or tripping. In addition to that, bonds can be made much more intense through these experiences. I think the fact that I met my partner via introdicing him to LSD one day when he was just a friend of a friend played some part in our connection to begin with, so it feels very natural to continue to use psychoactive substances to explore and build it.
