If you think this sounded vague because you can torture out a different "figurative" meaning if you pretend you don't know what words mean and just shuffle them around to produce something completely different, then i don't know what to tell you. You do you, i guess?That still seems pretty vague to me, as it could be interpreted literally, 'only have new sex while both of us are present' (which Lena almost certainly breaks to be able to offer up Louise), or figuratively, 'you can experiment, but we are together, ie. primary partners.'

And yes, Lena does break that deal if she has what's effectively affair with Louise. In fact, she explicitly wonders about whether that's breaking the deal and if she's cheating (but ultimately concludes that hopefully not, because "its with a girl so it doesn't count". Which is porn bullshit, but that's another story)
Can you provide any example when this happens? I don't really remember Lena or Ian who get in relationship to claim they aren't, or anyone really talking about exclusivity of their pairing.Every time someone else starts to refer to them as an exclusive couple, they always respond with "no, it's not like that", indicating they are not. So any excuse which states they are, it just bullshit.