I agree this development it a bit weird, but keep in mind it starts with heartbroken Louise coming to Lena for comfort. It's not exactly great moment for Lena to tell her "No, i just started dating Ian so let me call him and bring him here so he can fuck you too". Afterwards Lena keeps this between just Louise and herself probably because she doesn't really feel like her relationship with Ian is all that solid, and it takes quite a few chapters for both Lena and Ian to come to conclusion that yes, they feel like this is a real, working relationship they're in. At which point Lena does introduce Louise to Ian.I didn't meant non consensual as in forced, but there is no point in not telling Ian before sleeping with louise if Lena has agreed to experiment