My dad told me he was gay when I was in my late 30s and it completely shook me.
At first I was totally supportive, open and caring, and actually a bit proud that he'd trusted me to tell me. A while later it hit me like a shockwave and I actually went into real shock, which floored me.
All that lying, the pain he put my mum through over the years, all the way that my parents just didn't love each other and never had. It substantially changed my relationships with my parents, my understanding of my family, my childhood and my own beliefs. For example, it took so long in life for me to understand that real people can actually love and desire each other, and that romance and care and love weren't just fake nonsense.
It truly, really fucks with your head, and I don't think the equivalent is "what if he'd just been in love with another woman all that time", although I can't easily articulate why. It's a different sort of lie because it's not just "I fancied someone else", he's actually not the person I thought he was. So much of his whole life has been a sham and an act and a pretend persona.
I think if your parents haven't put you through it, you can't understand. Even I don't understand it. But I feel for the pp who tried to talk about this and got shot down relentlessly.