I definitely meant Brint in that specific instance. Despite him being dtf most people, if you get with him, then he shows constantly that you're #1 to him. Most of his threesome scenes (that are written by wsan, other authors don't usually do this), either have the PC being able to join in and Brint stating he can't leave his favorite out of things, or being able to focus the threesome on either the PC or Brint if you want (my fav example is the sleepy snek stuff). So for me, Wsan nailed his character. He was able to make a bro sir-fucks-alot minotaur still feel like he likes the PC the most.
And about poly vs open relationships. I def think the devs might throw around the word polyamorous when they mean, like you said, more of an open relationship or fuck buddies (at least for the above case with B). For me, with Brint's more loveydovey scenes, it def feels like he has an understanding with the PC that they're pretty into each other, even if/when they fuck other people who don't matter as much to them. And this is like I said, usually backed up by Wsan's writing because he gives you more choices in how you engage with his scenes.
It's why I have high hopes for Mino City, whenever we get more Brint scenes (plus I'm pretty sure it's been said you can either marry him or get him a harem, I think it's obv which one I'll choose lol). Not really into any of the other male npcs. Elyon has nice artwork but goddamn Tobs' sex scenes leave me bone dry. It is hard for me to get even a drop of arousal from reading his stuff lol.