To be fair the game actually has Lena mention her ongoing flings
during her talk with Ian:
Python:
# A - lena open
if lena_robert_dating or lena_mike_dating or v8_jeremy_sex or (lena_louise_sex and lena_reject_louise == False):
i "So is that a yes?"
l "It is... Kind of."
i "Kind of?"
l "You've been honest with me, so I will do the same."
l "I'm going through some kind of... \"experimenting\" phase right now."
# ...
i "So... Are you still interested in... {i}experimenting{/i}?"
l "I have... mixed feelings about it."
l "I have to admit it felt nice being able to do whatever without worrying about hurting anyone..."
l "But that won't be the case anymore."
Note, Lena's comment about "that not being the case anymore" can be interpreted as her being aware her continued "experimenting" will be emotionally hurtful, not necessarily that she's going to stop it (completely and/or immediately) :v
Technically it can be defined as "open relationship" during chapter 9 talk, if both sides have slept with other people, or if Ian is horny enough:
Although this shouldn't be treated as excuse for Lena to continue her "fun" with Louise, given she breaks off her other flings, and the talk clearly mentions "experimenting together" and not behind each other's back.