I honestly have no idea how romantic partnerships that aren't dom/sub normally work, but I *am* fairly certain that dom/sub relationships automatically being coded as romantic is a stopgap because the eventual dom/sub relationship mechanic hasn't been coded yet.
In any case, I have literally not once seen anyone get jealous, even those who aren't open/poly. I have several times seen that I "helped" someone cheat on their partner (in most of these cases I wasn't even aware they had a partner), but my actions and their reactions were exactly identical to an open or poly relationship.
Actually with terms or relation or friendship once, they could with one of my old save and more recent one. One or two different, a friend. Not a D/s relationship and low romance can feel that way, I have vaguely remember the one time I think only a hangout, an angry/upset; two friends or just acquintance met me mostly from one public hangout planned from the other, it particular got me surprised when it first happened and some instances that didn't standout in a single save from an ochre seed at that.
One other are, a romance, I have this one low that I chose to hangout, I didnt think through but i tried to get an another unnamed npc yet to flirt, the thing is, THIS certain cisfem friend got disappointed when i did it in the last recent update before this version. So it can be uncommon to have in this Non Dom/sub relations but to answer the former, poly or not, its either the part, if you could remember. Inclusive or exclusive after the poly trait.
That's gonna depend if they have it in them if one of them dominate the other in terms of romance/or lust % but fair game when you have them for now to max them up. That's gonna be on you're part if you know and remember when you got asked about it before starting with them. The collar terms, if you've have older saves or whatnot from the get-go as you only had the first one with you. The color are dependent on the style you got first time you did so it's harder to know if you don't have the saves atb each 100th saved to backed up. (Get it? Don't asked me but I've took a napped and I'm a bit airheaded to correction my shits), that's all.