Tonight I told my primary that I was pursuing someone else and asked if she was ok with that. She says “yeah”, then I see her vague-blog about it with “Fuck you, I wanted to be your only one.” I’m…pissed. Plz tell me how to handle this.

If any of that is a dealbreaker for you - the “fuck you,” the vagueblogging, the trust-eroding passive-aggression of saying something is okay and then being angry about it, or not wanting you to have other partners - leave this relationship.

If this isn’t something you want to end a relationship over, the best way to handle it is to have an open conversation: “Hey, you told me things were okay, but then I saw a post on your blog later that concerned me. Can we talk about this?” 

Make sure to discuss issues separately: first, ask whether they really are okay with you pursuing someone else. If they say yes, ask what the issue is with the blog post. If they say no, ask why they said yes when they meant no, and talk about how to prevent serious communication issues like that in the future.

Next, talk about the blogging: tell them it really hurt your feelings to read that, and that you need them to commit to not doing that anymore. Whatever you’re most hurt by - the “fuck you,” or the fact that they shared your relationship business with their blog friends instead of bringing it up with you, etc. - be clear about that, and why it was hurtful.

If they are apologetic and can commit to being honest with you rather than using passive-aggressive vagueblogging, you can go back to working on the larger issue of how non-monogamy is going to work between you two. If not, think about whether this is something you can tolerate or whether it’s a core issue in your relationship.