I think it’s more that “these people” didn’t used to ‘exist’ in their world: Sexual minorities weren’t talked about and were invisible for the most part. But since Stonewall and AIDS, LGBT+ people first decided and then learned not to hide themselves. Suddenly someone who you thought of as normal has ‘become’ gay. You obviously knew them well, it’s not possible they were hiding something from you, and there’s no chance your observations were incorrect - clearly, instead of you being wrong, something happened to them, something ‘turned them gay’ - it must have been the frogs, or music, or DnD, or something, it’s just impossible you were wrong!