Classic Lavender Marriage is the easy answer. In my wife's case she is deeply intimidated by pretty women so she has enough trouble talking to them. She couldn't flirt with one to save her gay ass.

Her not being attracted to me like that is fine by me since I'm ace. We're basically just besties on a permanent sleepover that we formalized for tax purposes.

Gay mom is from a basically all-female race of aliens. They're pretty much gay by default. But she and a few others got stuck here for a long time and she was kinda interested in men from time to time.

That's about as short as the answer gets lol

Because religious people have a lot more kids that non, so a bigger potion of people get stuck into a culture they think they have to conform to in order to keep their parents happy, also they convinced us sex of every kind was basically a crime against God, so we became sexually repressed enough to fall in love emotionally and not sexually. Not daring to even question our orientations until over a decade after marriage, parents die, kids grow up, we didn't raise them to be sexually repressed or afraid of being authentically true to themselves, and having a sexuality, which conflicted with our increasingly hateful and intolerant religion of birth, forced us to be honest with ourselves, abandon a creepy death cult, and accept who we always were. We finally understand why the sex was so terrible, the whole marriage, neither of us was interested.