To answer your last question, yes. If by "interacting in terms of a relationship" you mean "having sex." Which you may not. I can be alarmingly obtuse at times.

But if the interaction is in any way in the context of a relationship, then yes. The / can still be present, even if it's not the sole point of the fic. However, I'd say that was entirely at the author's discretion. As is everything, really.