Interesting. "My boyfriend is so good and he's hanging out with different hot girls all the time that I feel inferior to. What should I do? Maybe dress in a different way? Try something new in bed? Try watching some porn to get new ideas to spice it up? Have more romantic dates to make him see that I'll never hurt him? No, cheating sounds like the best option. Once I'm slutty and experienced enough, no way he'll look at other girls!"
I'm not really an expert in psychology, and while technically there have been cases like these, in order for that to be possible, Holly's insecurity would need to be way stronger than her love for Ian. Because cheating on your boyfriend, who is still struggling after his last heartbreak, seems like a sure way to end that relationship in the dumbest way possible. Even if she may feel insecure intimately, I believe she doesn't think that cheating is something a loving girlfriend should do. And Holly may be gullible, but she's not stupid and can see several steps ahead. After all, she orchestrated the whole "Oh no, my room has only one bed so we'll have to sleep together" move. And on top of it, she's an avid reader. I imagine she read countless romance stories (good ones) and knows the most common relationship-ending factors like cheating, jealousy, lack of communication, etc.
If anything, if she's indeed this insecure, I think it would make her open to share Ian with other girls instead so he'd feel sexually satisfied with them and emotionally satisfied with her. You can already see this in the way she treats her relationship with Lena. When she learns about Louise or Ivy being side-hoes, she doesn't feel jealous (inferior to Ivy, sure) and is instead willing to share Lena with them and simply be happy that she's willing to spend time with her at all.