I don't think the homosexual experiences are a red flag by themselves. I think what is more of a red flag is not being sure of her sexual preferences.
My stbx had told me that he had sexual identity issues when he was younger. I didn't think anything of it because it was when he was younger. When we had been married 15 years, he came out and told me that he was bisexual. He had a bisexual experience at that time and then decided that he wasn't bisexual after all. A couple of years later, another opportunity for a bisexual experience presented itself and suddenly he was bisexual again. This time, he went so far as to come out to a bunch of friends and said that all of his issues were related to him being confused about his sexual identity.
The funny thing is that we are both seeing other people. He made such a big deal about wanting to explore his bisexual side yet all he is chasing is women.
Sorry, maybe I expressed myself in a wrong way. I also didn't say that homosexuality is a red flag. When I was refering to the red flag I meant her and her friend in bed, when I didn't know that she has sexual identity issues.

At that time, to me it was weird that two girls sleep in the same bed, especially if one of them is lesbian. Like I said, at that time I didn't know that my ex had lesbian experience. Who knows, maybe it was a 'girl thing', without real action. I will never know.

But it is not odd that they don't have clear sexual identity because they don't have any image of themselves. In any aspect.