Opiates are for foreplay lovers. Girls guide to Sex with opiates.
Both my BF and I use Opiates, both of us change drasticly while we are using, we both get extremely horny and lovesick affectionate to the level that would normally nauseate us. We are all over each other, at least for the first few hours. Sex for us is great, He struggles to stay hard after about 30 mins but he can repeatedly get it back up after a little friendly encouragement. Maybe we enjoy it more because we understand that drugs always negatively affect men and we use that to our advantage. If he can't cum then we just "play" for hours and do anything that makes you feel good. Neither of us can usually cum while using but sex feels like a constant orgasm anyway so you sorta don't care as much. You just fuck until your out of breath then you have another shot and do it all over again.
I don't feel bad if he doesn't come, he doesn't feel bad we just laugh and I suck him off until he's a dribbling mess on the floor

You just have to love foreplay and enjoy making your partner horny, even if they aren't giving off the usual sex vibes it doesn't mean you can't try and make them want it... It works for both guys and girls ( obviously easier for woman) but its not impossible.
And don't give up if you can't maintain a hard on the entire time. If your GF has an issue sucking your dick after vaginal sex then find a new GF, mix it up.....if your struggling to stay hard for sex just go back to foreplay.
That's about it for sex on opiates, for 90% of moderate to high users once you stop using and withdrawals kick in you will not feel like sex at all....But you should still try and have regular sex when your not using, it breaks drug routines and it actually helps with some withdrawal symptoms that I HATE ...(the hand spams and shakes/ body aches and fevers and insomnia/irritability) It takes your mind off the pains.
(I can't vouch for couples where only one of you uses...Im sure that would make things a lot more difficult)