I agree. I am not Ian or Lena, neither is anyone else playing. So the characters have their own pre-established values and interest, (those that I suspect mirror the author's). For example we can't choose for Ian to have a threesome kink, he has one in every playthrough. We also can't give him a hardline stance against Lena sleeping with women prior to the threesome. He says he does not know how to feel about it and says he wants to be included in the future. Again I think later when we get to play as him I think there will be an opportunity for us to decide how Ian reacts, whether he says to always be included, allows Lena to sleep with Louise on her own time, or if he forbids it entirely.
All of that being said I still think Lena cheated on him given the facts of the situation. If they agreed to experiment. They explicitly state to experiment TOGETHER. And in a few scenes leading up to the threesome Lena and Louise have sex without Ian. If after they got together and they immediately invited Louise for a threesome that'd be different.
About it being different if it's the same gender. Cheating is subjective and is defined by whatever boundaries established in a relationship. Ian and Lena never agreed to be open, so the implied relationship is that of a closed one. Lena lucked out because Ian didn't immediately consider what Lena did to be cheating, certainly not how he'd have reacted had it been a guy, but Lena had no reason to know Ian would react the way he did and in her internal monologue she repeatedly thinks that it could be considered cheating.
In other words by definition and the their relationship boundaries Lena cheated, but Ian might not care because threesome.
