People are fighting their corner like what's going on in the OPs relationship is a reflection on their own. In the immortal words of Cameo's Rigor Mortis:
https://www.youtube.com/watch?v=Wz13jZaDOPc&t=2m35s
As long as everyone involved is on the same page and open about their intentions and desires, then no one should have a problem with someone elses definition of a relationship. People in open relationships aren't sluts looking to 'have their cake and eat it' and people in monogamous relationships aren't prudes looking to emulate heteronormative roles. I find both these takes extremely patronising.
My take on the OP's situation is that the honeymoon of moving in together has worn off and that's probably triggering feelings that already exist. OP you admit to enjoying the "thrill of the hunt" and flirting, the new - this isn't uncommon. But you must talk to your partner about it. You don't sound like you want to break-up at this point - even though some people might be foreseeing any opening up of the relationship as a stepping stone to just that, know one knows that, it could have the opposite effect. But not communicating about it probably will result in a break-up because your resentment will probably only grow, and your partner will sense that distance (if he hasn't already). Be honest, maybe he feels the same way? That it's got a bit stale. You don't have to jump in to the deep end right away, you could just try discussing fantasies you each have that you haven't yet explored together and take it from there. If you do want to be together, make it work within what else you want to do in life, both of you. Don't sit him down with the "I've got something to tell you..." stuff, that gets people worried and on the defensive right away. Have a night in with a glass of wine and chat about stuff as you watch a movie or something, don't make it bigger than it is, because you haven't even made up your mind about anything yet, you're just exploring what's in your head that involves your partner.
We all make our own rules in relationships, none of us are bound to any pre-existing roles (either poly or monogamous) unless that's what we want. "What's good for you, might not be good for me".
You do realise that the people saying "closed-minded" are not calling out people who are happily in monogamous relationhips, right? And that they're calling out posts like "break up now".