I think that most people are complaining because the reaction is outside their control. Like Lena choses to act that way herself despite whatever she did before, so in some playthrough comes as a hypocrite or out of character.
But i think the reason could be explained this way.
I think the problem here is not about Ian sleeping around if Lena is doing the same.
Lena's reactions comes that way because Holly is her friend, and not only that, its her common friend with Ian. Ian is the one that opened their friendship, so he, fooling around with both it's interpreted as a recipe for future disaster between the three.
So Lena doesn't want to lose Holly's recent friendship just because of Ian, so she stops hooking with him for that sole reason, and her being dissapointed seems to be a frustration to being forced to take that decision for the sake of keeping Holly's friendship.
Seems similar to Alison-Emma in a way. I think, in one route, that Alison called out Ian for that same reason if he slept with Emma (Accusing him to try to sleep with all the girls inside their friend circle) while her being "OK" if he sleeps around with Lena or Cherry.
Once Lena decided that they are not hooking up ... that's another argument, if she choses to sleep with Holly once they are not hooking up, that's just a bland excuse to not overthink that situation, so she doesn't have to feel guilty about that.