Go outside, throw a rock into a crowd, and you'll probably find any number of people who's insecurities are stronger than their love for their partner. You'll probably also hit any number of people who have also reasoned themselves into doing the dumbest things possible. A lot of relationships end from self-sabotage, whether intentional or unintentional, and it certainly wouldn't be the first time that someone inexperienced and insecure fell into the wrong crowd, took the wrong advice and ended up making a mistake.
Because sure, it could push her towards the sharing route, but that's not the only reaction that an insecure person could possibly have. And to be clear, I didn't suggest that this was something she should come up with on her own, but something that should be influenced by corrupting Holly through a slutty Lena and Ivy. Like I said above, wrong crowd, bad advice. No matter how many books she's read, she doesn't have the real life experience to back it up and Ivy and Lena clearly have a lot of experience. It's not hard to fall into the mind trap of thinking they might know more about this than she does, even if they're corrupting her for their own enjoyment, rather than thinking of her or Ian's well-being.
If Holly is going to be this mary sue character who can do no wrong, who always makes good decisions, even if she's surrounded herself with people making bad decisions and trying to influence her, that's boring and not realistic either. Having a boyfriend shouldn't make her suddenly impervious to manipulation and corruption.
