Cheating is done when you sleep with someone your partner doesn't want you to. For most people that means anyone that's not your partner. In open relationships it's not cheating to sleep around, but would be cheating if your partner said not to sleep with someone specific and you still did. For certain couples, both fictional and in real life, it's not cheating if the woman sleeps with other women. From both Ian and Lena's perspective, it's not a big deal for Lena to sleep with Louise. That doesn't give Ian the right to sleep with Louise, threesome or not, the code of ethics dictates no sleeping with someone your girlfriend doesn't want you sleeping with, in Lena's case and most 99% of other women, that means all women. If Ian doesn't like these rules, he shouldn't be in a romantic relationship.
And if we want to say Ian should be given the opportunity to be upset at Lena for sleeping with Louise, I would agree, but the game does not have infinite choices. And certain things need to be hard coded into Ian. The fact that he is into FFM threesomes is also hard coded. Sucks, but we can't decide his stance on lesbian sex during hetero romance.