bm whose so isn’t invited to wedding...

I agree with Sylvia here! And while I agree that this could impact the relationship between your BF and the couple, I personally think (having been in a kinda similar situation myself) that it could easily impact the relationship between your BF and yourself also.

Here's a quick recap of my situation: started dating my then-boyfriend, now-husband a few years back. Six months in, I got a save the date with only my name listed, but I *assumed* (yeah, I know, I know) that my BF would be invited due to the social unit thing (the couple definitely knew I was in a relationship), and especially because it was an out-of-state wedding. Now, I was only a guest, not in the BP. But when I got the invitation and it clearly only listed my name, I was in a very awkward situation. I had been talking up to my BF how fun it would be for us to attend this out-of-state wedding together and make a weekend trip out of it! And he was off-put when I told him he wasn't invited alongside me. Now, my situation wasn't as awkward as yours because I wasn't in the bridal party, and the bride didn't flat-out tell me that because I didn't fit into one of her designated categories (married BP or doesn't-have-to-be-married guest), that my relationship didn't count as far as invites go. And I was not close enough to the couple to discuss the topic, so I just declined the wedding invitation. And I told my BF that it was not a hard decision....to choose between a friend from high school (who doesn't respect my relationship) and someone with whom I was in a committed relationship. I understand that it's more nuanced for you, but I just wanted to share my (lots more cut and dry) experience.

So, having been in a similar situation, I personally think it's important that you "go to bat" for your BF/your relationship and have a respectful conversation (as Sylvia mentions) about how you would like to bring your BF, or otherwise you'll be forced to step down and attend as a guest. Respectfully, speaking as an outsider with no skin in the game, I think you're being a little too understanding towards the bride here, and giving her a little too much benefit of the doubt. The PPs and I agree that this bride's behavior is not standard etiquette, nor is it logical. Let's just assume for the time being that the bride doesn't realize how rude this is, and she also doesn't realize how small ONE invite is, in the grand scheme of wedding planning. If the bride can't understand your point of view after a respectful conversation about it (aka "words"), my personal opinion is that the "action" of stepping down is the only remaining solution.

Best of luck to you going forward!! If you're so inclined, please feel free to let us know what ends up happening!