I think it's definitely a psychological effect the drug has, OP, in addition to any unwanted physiological effects….
Never had sex on meth.
But I've noticed the SAME exact thing while high on Benzos. Aside from the laziness benzos created during intercourse, they also make me less perceptive to my partner's emotions/feelings. Almost to the point of complete disregard. I would much rather kick back and let her do all the work, but additionally, they give me an almost cold, indifference to my character during sex. It's almost like i power trip on them, like "get on your knees and pleasure me with you mouth while I sit here and enjoy it like a king."
I love blow jobs, but I like them in a very different way on benzos. trippy stuff.
Also, I have noticed after some heavy opiate abuse, it has been more difficult to maintain erection through intercourse than oral… Not so much psychological here though, most I think it's just the blood getting sucked into the penis or something.