Unless:
1. You choose the path in which they agree to have a relationship in which they can both "experiment". (I saw this in cheat mode)
2. If one finds the intense hypocrisy and deception to be erotic. (I am assuming that they can choose NOT to experiment, agree to have a loving monogamous relationship, and then immediately cheat on each other. Is this actually true? I have not seen it yet.)
On the open route, I've played them so far as faithful. I want them to mutually decide to experiment.
But on the 'monogamous' route ( which I play as the cuck route), Lena can go straight to Axel's apartment for a photo shoot, and if she's been obsessing over Axel, and she allows the photoshoot to go too far, then unless she has will she automatically cheats with Axel. And if Ian goes with Cindy to her photoshoot, and they've previously had sex, he wants to talk about that with her, which can lead them going to her apartment and sleeping together.
So, it looks like I did it the opposite way to you. On the 'monogamous' route, Lena can cheat with multiple characters, Axel, Mike, Mark, Jeremy, Marcel, Jack, Robert( possibly), Holly, Ivy and Louise, whereas Ian can cheat with Cindy, Alison, Jessica and Minerva, but not Holly, Emma or Cherry.