a female reporting here. opiates make me horny- all of them. come up, come down, doesn't matter, there is typically always a period of time when i'm high on an opiate that i really want to fuck... until i've hit about day 4 of a binge and then sex or physical contact is that last thing on my mind. as long i don't binge increased libido is a constant side effect.
now, when i was younger and first starting exploring drugs, my (then) steady boyfriend who was also an opiate enthusiast shared the same effects on libido that i did. if we ended up having sex, it was possible for him to cum on the come up, but impossible on the way down- personally i would attribute it to the way dopamine is utilized in both the opiate high and the high of orgasm.
since that time i have found it far harder to help a male companion dose and then subsequently have great sex, the timing always seems to be off in a little way. i think in the end its about knowing the nuances of your own libido and your partner's while in an altered state, which is easier for some people than for others. none of my sex partners since him have been avid drugs users so i've never been able to learn how their bodies react to different things with mine.