That's the impression I've been getting of this beach event for quite a few chapters now. We're probably going to be looking at the relationship fallout of several of Lena's and / or Ian's dalliances. Notice how most of the LIs aren't going? It's probably for Perry and Wade to judge Ian and Lena. Holly is coming because she's the third main character. Emma is going because Perry will find out the girl he has been crushing on has either had sex with Ian, or likely cement his own relationship with her.
I promise you beyond a shadow of a doubt, whatever happens at the beach event, Ian will suffer disproportionately for his indiscretions compared to Lena.
I didn't say it was impossible, and what you have described it's basically the only way among all the possibilities. Even having a relationship only with Alison, Ian can still be an asshole insisting with anal sex, refusing to use a condom and the bimbo stuff.
Well yeah you CAN, doesn't mean you should, I mean why would you want to? You say Ian treats Alison like shit, I say nuh uh, my standard playthrough involves him treating her like a princess and avoiding the other slews, you say well he still has the option to treat her like shit...?
What exactly are you saying? You want her choices to default to cherubic behavior? Or she doesn't have enough "good" options? I'd probably agree with that, but that's the case for almost all the "good" actions in this game which is clearly sleighted towards a bully/slut-em-up/cheating dating sim.