The about labeling romance based on sex/gender instead of orientation raises another issue that I've been exploring recently: cross-orientation relationships. There's almost no discussion of it, and that causes problems. Cross-orientation relationships can happen when someone with a wider span hooks up with someone of a narrower span, like bi/gay or bi/straight. It can also happen when someone falls in love with a person of incompatible orientation, but they don't want to give up the relationship, so they try to retune it into something both of them can live with. Frex, if a lesbian falls in love with a straight woman, they may not be compatible sexually but might be able to adapt into a queerplatonic relationship. Ace with anyone sexual is another example.
I think my favorite cross-orientation relationship right now is Socket and Fortressa. Socket is a lesbian. Fortressa was previously involved with men but decided to abandon the whole love/sex thing -- and I don't think she's just celibate, I think she somehow hit her sexuality with a wrench until it crumpled into a motionless lump. It certainly isn't the same shape it was, and she doesn't respond to men the same way anymore. So. They started off with Socket in love with Fortressa, who had sworn off love, so Socket didn't say anything about it and set about becoming friends instead. That has actually worked pretty well, except that now the cat is out of the bag and they have to deal with that realization.
I think that, if people don't even realize this is an option then they miss out on a lot of opportunities, and if they stumble into it by chance, it can cause tension because the patterns don't match exactly. Just as a M/F relationship requires accommodation of the sex/gender difference, orientation differences can take some adaptation too. That's hard if you don't know what the heck you're doing.
This especially comes up with bi folks, because while the inclination toward bisexuality is common, the number of people actually identifying as bi is a lot smaller. Many bi folks join up with someone(s) who is gay or straight. And some of the problems in bi erasure come precisely from that lack of awareness about cross-orientation relationships; without it, people have a nasty habit of thinking that being with someone of the opposite sex makes you straight, or the same sex makes you gay. It's a case where labeling can make things clearer: "No, I'm not gay. I'm a bisexual man in a cross-orientation relationship with a gay man."
The question of combinations is another one. A majority of bi romance is written as MFF or FMM. In effect that merges bi and poly romance. This is a problem because many bi people are not poly, and many poly people are not bi, although there is a fair amount of overlap.
I have some characters who are both bi and poly. I have some poly families. I actually have more poly families than people may notice, because not all of them are tied together entirely by sex, and I count poly based on strong, lasting relationships rather than just fucking -- it's about who moves through life as a social unit. So if you mapped out the lines there would be some sex, some romance, some queerplatonic, etc. within a poly family, and that actually seems to be the norm based on poly families I have known. Not all of them are a blob of everyone-sexing-everyone.
Plenty of my bi characters are in exclusive relationships, though. Stan and Lawrence, Danso and Noah, they're in stable binary relationships. So as usual, I'm doing my "tell ALL the stories" thing.