You and I are on the same page (43 is it?) completely. I am straight. I have dated more bi girls than straight ones over the years, and also never thought of them being with a woman as cheating (some of them did and were upset that I was okay with it - weird), but had jealousy/possesiveness issues when it came to other guys - "why do you eed/want him if you have me" stuff.
When my current longest term relationship started, I was mystified by her assertion that I could be with other women and it wasn't cheating in her book as long as I didn't leave her over it. I won't explain here, it took me months to get my head (mostly) around it (still not ALL the way there). I hated the hypocrisy that would be there if I "took her up on it" as I was not nearly as comfortable with her being with another man (which she didn't care about, since she doesn't want another man, only girls).
As I've watched our "unicorn" grow into our relationship, it's opened my eyes a little to the possibilitythat I could learn to respect my GF(s) having a romantic interest in another man, so long as it didn't detract from us - much as neither Violet or Anne cosider each other competition. It wouldn't be quite the same as they're both bi and very close to each other, but I can grasp it.
As for commonality of different arrangements - based on the boards here as a representitive sample - which may or may not be anywhere remotely near accurate - a "V" with one woman and two men seems to be the more common arrangement! Exclemation point for surprise, not objection, lol.
But there are other "V"s, triads, quads, and more here, some of them all of one gender, others 2+1 or 2+2 or 3+1 or more...
It's generally whatever works. As long as all partners are open and HONEST, anyting CAN work.