Context still matters. Her posting it on instagram is different than posting it on a forum full of your internet bros or w/e. I'm not saying it's "relationship destruction" worthy but I could understand that upsetting her more than the other stuff.
That's a fair way to look at it. I would assume for most dudes though it just feels like it's different(less of a serious thing) for us than them. Double standard, yeah, but men and women are different and it's more typical for guys to BS about these sort of things.
Did she ask you to stop posting? Was it an ultimatum kind of deal? Or was this something you decided yourself you needed to do for her?
I understand there have to be compromises made for a relationship but it does seem a bit overly controlling if she wants you to stop posting.
I also feel like she is a bit at fault to some degree as it was kind of a privacy breach for her to search your post history. I can understand the curiosity and temptation but I personally wouldn't do this to someone.