I'm a lesbian. I was born in the 90's, and I didn't even entertain the thought I was gay until my early 20's, and that's only now from living in a time where there are lots of gay people on TV etc.
When I was at school, there were very few gay people in the public eye. There was one gay boy at school. I didn't know any gay couples. I had boyfriends, but never truly fancied them or understood it really. I just did it because it's what everyone else did. I just thought that's how it was, and that's how everyone felt. I then went through a phase of thinking I was A-sexual or that there was something wrong with me, because I just didn't get it when other friends talked about sex.
Then I went to university where most people are liberal and people experiment. I met gay people, and for the first time, it felt like an option. Everything started to make sense. But it took a long time for me to accept it and tell people. Even in modern times, people do still sometimes double take when they see you holding hands with another woman. And coming out to my parents, who were actively homophobic, was damn hard.
What I'm saying is, your mum may not have known. For your mum growing up, being a lesbian definitely won't have been an option, and society was actively far more homophobic. It's only really in the last 20 years we've progressed a lot. Don't forget the hysteria around Aids was in the not so distant past. She may have had a sense something was 'off' with her relationship, but may not have known what.
But that's not to say she didn't love your dad. Love and attraction aren't always the same.
It's hard to explain if you're not gay, but it genuinely just wasn't an option for me growing up, and it certainly won't have been for your mum. Straight was the default. It's not as straight forward as 'you always know'. I didn't. A lot of middle aged gay women have been married to men.
It's totally okay to feel sad/angry/hurt, OP. Your feelings are your feelings. But I'd also encourage you to chat to your mum and her experiences with her sexuality, if you have that kind of relationship.
Also people on this thread acting like being gay is no big deal have no idea. Homophobia is still alive and well, and it certainly was in the past. My dad told me I was vile when I came out, and expected me to not hold my gf's hand in case any neighbours saw. I'm still very careful who I mention my wife to die to negative reactions, and coming out in the workplace for me personally is very stressful.