Like BloodyMares says, there is quite a bit of exposition through the whole game about Lena's reasoning when doing things. If the "worst thing" that ever happened in her love life was cheating, and she's constantly talking about how much it hurt, it doesn't make sense for her to cheat. She always talks about her modelling in an artistic way and the way she reacts to most things leads me to believe she's accepting of other peoples decisions, she just doesn't want them for herself.
There is also another piece to consider: if we think how Ian is portrayed and we think of the reasons that makes Lena like him, that doesn't sound like a girl that just wants to snort cocaine and fuck around with a bunch of different people.
I'm not entirely sure, but from playing the game while using the walkthrough I got the idea that the options that are needed to take for her to go down those paths are pretty off-character.
In the end, it's all player interpretation. A lot of people come to this game after GGGB for the corruption and slutty-ness, so they tend to defend that type of behavior. Others found both games and wanted fairly wholesome experiences (I myself look for games with a chill vibe that end well for everybody).
If your Lena was slutty from the beginning, you think that all the choices makes sense. If my Lena was always viewed as wholesome and prude, I'll look at the same choices and think they don't make sense. It's a matter of what is your favored perspective.