I think a lot of the reason why Lena / Holly aren't official is because of Holly's concern regarding her parents reaction. Could be wrong, but that's always been the implication to me about why Holly is so shy and reserved most of the time. They can have sex and her parents can be none he wiser; while I suppose that could be true for dating, it would be a lot harder to hide because of social media and it just being a general pain to hide a relationship.
Given her conservative background that would make total sense, but she just doesn't quite act the way that I've known girls like her to do. It's not really been a topic between her and Lena either.
I mean, Lena and her can kiss somewhat carelessly outside of the club, for example, when saying bye at Ivy's birthday. Now, the likelihood of mommy and daddy walking by is of course slim to nothing, but someone who is truly worried about that sort of thing, y'know getting caught emulating sappho and all that, would usually look around or perhaps hesitate before diving in for a good ol' public tongue-wrestle, or even express a swift note of worry after the fact.
Holly doesn't really ever express a direct conflict when it comes to her romantic life in a same-sex capacity and her parents, it's not mentioned when she talks her first girlfriend either for example, the one who abandoned her for a new social circle.