It seems unfair that men have to accept every single sexual act, thought, etc. of their girlfriends or wives or else they are "slut shaming." How is that healthy? So he has to swallow every sexual impulse or action that his girlfriend has or he is a misogynistic asshole? Not fair! If she came to him and said, "hey, I am really into this blog and it excites me to put pictures of myself on there, but I wanted to see what you thought first" and then he lost his shit and called her a whore, that would be slut shaming.
the thing that's not totally clear in the first letter is whether or not she was up front with her boyfriend about the blog and what was on it. she says he 'discovered' it, which implies that he didn't know about it before, but then later says that he'd tease her about it, which implies that he did. i think that's the key difference, because the core principle of any relationship is honesty. nothing- underwear photos online, flirting, kinks, sex with other people- is necessarily wrong if it's undertaken honestly, but any one of those things done clandestinely can be a serious breach of trust. it wasn't just being 'open' and 'expressive' if she couldn't or wasn't expressing it to her boyfriend, it's sharing a part of her sexuality with the anonymous internet that she wasn't sharing with him, and it's totally understandable that he would feel confused and hurt by that.