Man, this feels like you're the one setting the rules on how Lena's psychology or a woman's psychology must work. I'd guess Eva being the talented creator she's proven to be and being female has consistent grip on what would be consistent with a female character, especially considering only she knows what is Lena like.
In any case I feel you-re assuming like those 3 scenarios are the only possible ones where a girl like Lena could do what she does, but you're narrowing her personality through your point of view. Maybe through your morals or understanding of women it doesn't make sense, but maybe girls can be as horny as guys and enjoy hooking up with a bloke they met in a club as much as you'd enjoy having a one/night stand with a girl. I'm sure you wouldn't necessarily considered yourself a bad friend for ending the night in bed with a hottie rather than with your drunken friend, or that if you like another girl you would never want to shag this one, or that fucking with some other girl has awakened your inner slut -or stud? lol
All I'm saying that this is how the author envisioned one of the possibilities of her character, and we can have our opinions but we don't know Lena's mind or any other girl's mind, and people can be all kinds of ways and act in seemingly uncharacteristic ways. And this doesn't seem that hard to believe to me
You also can't just ignore the semblance of logic that should be present while writing characters or scenes just because Eva Kiss did something a certain way (and her gender has nothing to do with it, it's sexist to believe otherwise). It's one thing to have an idea, and it's another matter to present and sell that idea. It's all about execution. It's not a secret that she's constantly pressed for time, and it's not too far fetched to guess that she failed to properly characterize Lena during those scenes precisely because she was in a rush.
I, for one, think she managed to write Ian way more consistently so far, and he's a male character, so no reason to make it about gender. I don't have a separate moral understanding of women. I just know that Lena is presented to be a person who dislikes cheaters. The problem with Mike's route is not that he's just a random dude for a one night stand. The problem is, Mike is someone's boyfriend. Just like Axel was to her, and she hated his guts for cheating. No matter how horny she is, it's not easy for a person to go against their morals. Even for Ian, it's not so easy to go after Cindy, his guilt is eating at him constantly, but it's also hard for him to deny the attraction. With Lena and Mike, while the attraction is there, it's not like she's so desperate to get laid to stop caring about the fact that if she tries to seduce him, she'll make him a cheater, and thus she'll end up causing pain to another girl, a pain that is all too familiar and still fresh for Lena. If Mike didn't mention his girlfriend, most of my problems with Mike's route would disappear in an instant.
I admit, that I might be wrong in the assessment of Lena's character, but WE DO KNOW Lena's mind. Every update, we get to learn about her personality through inner monologue, it has been this way for 5 chapters already. If I have a certain belief about Lena's character, it's only because of what Eva presented to us in her storytelling. And it's hard for me to fit her behavior with Mike to the rest of Lena's personality that we do know about.