Drugs are not an issue. If, on the other hand, they are used as an accessory to have sex with someone while they are unconscious, then it might be rejected.
I never understood. What fun is it to have sex with someone who is unconscious? I don't even like to have sex with someone who is drunk. I started to write two of my characters getting drunk and going for each other and went, "Naw, can't do that." But i have two charactyers tokin upo a joint right now, i willsubmit that one and we willk see how it goes.
I've seen a few stories with drugs used for coercion/non-consent (definitely not my thing, though), so I'm sure that two characters smoking weed together would be fine.