Have an aunt who has only ever lived with women. I was always told it was her roommate. Everyone refers to her as a roommate. They BOUGHT THE HOUSE TOGETHER. Have dogs together. Raise the other ones kids together.
When I was like 17, and openly gay, I straight up asked my grandmother if her sister was a lesbian. She said no they're just roommates and got super specific about it. I asked my aunt a couple weeks later when I saw her and she went "Well, yeah. Do I not wear enough flannel?"
That's still missing some of the nuance. The way you're phrasing it paints a picture of having known at some point and deliberately burying it. This can also happen, and is called suppression. But it's also possible for this to happen without any conscious awareness from the beginning, which is repression. In that case, there's no "starting" to believe it, because you never had anything else to believe to begin with.