I actually wonder about it, in the sense that when Lena and Ian meet in the game's prologue, she complains about her boyfriend being angry with her for making him jealous, and how they fight over how possessive and controlling he is. While we presume that the boyfriend in question is just an asshole (and well, he is) i'm not excluding possibility that Lena was actually quite open to flirting with other guys/girls, and that Axel's complaints and jealousy weren't entirely unwarranted.
In other words, Lena's shock/trauma could be very much coming from the fact she was the one who was cheated on, and less so from how she feels about cheating in general. This would also explain to some extent why she never stops to consider what she's subjecting Mike's girlfriend to, when she goes all out to flirt with him and seduce him. And how her internal thoughts when she cheats on Ian are all basically "i know i shouldn't be doing this but i just don't care because i'm so damn horny".