According to Eva "Open" relationship only counts as mutual experimentation - i.e. threesomes with a third character when both are aware and willing to share. Cheating behind one another and then pretending it's okay because open relationship would still be cheating. That's why Ian can't get naughty in the bathroom with Emma if he committed to Lena (even if experimenting), because it's going behind Lena's back. I'll ask Eva why Lena can cheat on Ian with anyone but Ian can't.
I've always assumed that the "experimenting" relationship was phrased intentionally vague to give room for misunderstandings between Ian and Lena—to blur the line of what's cheating or not. If that's not the case, I think that ideally could have been a bit more explicit in the chapter 9 relationship talk.
Considering how Eva views the open relationship, I think it's reasonable that Ian can't have sex with Emma in the bathroom, as Emma knows them both and Ian isn't on drugs like Lena is in her bathroom scene. Though I think the scene with Emma would've benefited from a bit more interior monologue from Ian about it. I've only played the scene once, so maybe I'm not remembering it correctly—but as I recall it, Ian pretty much chalks it off as something that can't happen now, even if he's tempted. If he's already cheated with Cindy, I think it would be reasonable that the situation came off as an actual dilemma between staying loyal to Lena and indulge in the temptation of Emma's sweet ass. As in: It doesn't happen, but the thought is so appealing to Ian that it's established within the realm of possibillities.