Ok, I've had this problem for some time, and yes, I've tried the usual things, like lowering my resolution, removing mods, etc, all to no avail.   So if anyone can tell me why my companions disappear during sex, I'm all ears. It's *only* during sex, and even then, not all the time, just sometimes.   Oh, and it makes no difference if they are custom NPC's or game NPC's.   ..........Maybe I've overlooked something.