@Sanitisethat I'm bi too so I wouldn't necessarily mind, but most women aren't.
I’ve also had a very close friend transition in the past and it was surprising how quickly we all came to terms with the change and accepted our friend’s identity as completely normal.
It's different if a lover (or parent maybe) transitions, at least for some people. Many people go through a process akin to bereavement, as the person they knew has died. They even don't have a husband anymore, or a child a dad anymore.
Imagine you are straight and your husband goes from looking like a young Ross Kemp or whatever, to living as a woman and maybe even trying to act feminine. You're straight. You might still like them as a person (as long as they're not a self-absorbed twat about it all) but you're unlikely to fancy him as much when he's transitioning/living as a woman.
So if a straight woman stayed in that relationship they'd be condemning themselves to a whole diifferent sex life they didn't sign up for, which they often describe as grossing them out, or no sex life at all, and no-one should feel they have to live like that.
And that's on top of the whole thing of their sexual identity being supposedly changed in society's eyes, so they're seen as 'a lesbian' without them being the one that decides it.
It's like their own identity's taken from them too. Then the ex-bloke might go round calling himself the children's 'mother.' So he's even infringing on her identity as the mother that bore them.