I've been following this thread but haven't weighed in until now.
MC might be bi, she might be hetero, she might be pan. I don't really care and I can't speak for her. She seems happy, which is a good thing.
I have no problem with anyone choosing a relationship that makes them happy or defining their sexuality in a way that makes them feel secure (so long as everyone concerned is a consenting adult, of course).
I do take offence to individuals - and I hate to say it but it does seem exclusive to this younger generation - who seem to think that they have reinvented sexuality whilst minimising or erasing experiences that I and may other have had. Bisexuality, flings, crushes, experimentation etc is nothing new. I've had a conversation with a younger acquaintance who identifies as pansexual and gender queer, questioning why my life experiences are different from hers and noting that I've never felt the need to label myself. Her only argument was that I'm old (42!) and just can't understand the way her generation are looking at the world, seeing individuals, not genders and not setting any boundaries in their quest for a relationship. I dunno, it still sounds just like me in my 20s but clearly I'm just not woke enough.
For me, this is just another example of the minimisation of my existence, experiences and opinions that I have encountered since turning 40. it seems so easy to write us off as "just" middle aged women with nothing to offer here. That's why I get pissed off with the likes of MC and her "redefining" of relationships.