I agree with a lot of the posts here in support for your mum, whether it goes against the grain or not.
My mum told me many years ago, that on her wedding day to my dad, she didn't want to go through with it, but didn't want to let her dad down, as everything was all arranged and he was bursting with pride.
I wasn't planned, they didn't want children, but they had me 8 years after they married and they separated 8 years after I was born.
They were two different people, with totally seperate requirements from a marriage.
I think my mum did love my dad, she just wasn't in love with him. And I could see once he left and got on with his life, that my mum was so much happier. There were faults both sides, but I loved them both equally. Right up to and beyond both their passing. (Both heterosexual, btw).
My point is people can feel tied into a marriage for all sorts of reasons. In this case, add into that, the underlying situation of feeling like you're not who you are truly meant to be, it must be an awful feeling.
However hard and hurtful it is for you all, a year has passed and it sounds like your mum is happy. She hasn't duped anyone, things like this weren't expressed at the time your mum probably got together with your dad, due to shame, generational misunderstanding, fear of losing family and friends, the list goes on.
At the end of the day, she's your mum, she's happy and her sexuality shouldn't be an issue. Your parents, as people have rightly said, are people in their own rights.
And if you had a happy upbringing, while the family were a unit, that means more than anything.
I know it's hard and easy for me to say, but try and stay neutral and see and treat your parents in their own light, rather than just a mum and dad.
Hope you can get past all this, wishing you the best 🌷