As for not being able to mess around with Holly when Ian had nothing to do with Holly, she doesn't want to confuse Holly's feelings and make Holly feel like a homewrecker. Holly is not a girl for casual hook-ups, she's very sensitive to being rejected or toyed with. She opened up to Lena that she had both a boyfriend that was a dick to her (that's why she gets mad at Ian for sleeping with Holly if he didn't have serious intentions), and a "girlfriend" who betrayed her. Lena doesn't want to betray Holly and risk their friendship with her by hooking up with her while she's also hooking up with Holly's crush. Holly still has feelings for him and that's why Lena doesn't want to complicate things between the three of them. It would be all too awkward for Holly to interact with Ian because both of them would have slept with the same girl. Jeremy is happy with sharing girls with Ian, for Holly it's uncharted territory.