One of the most common biphobic narratives is that the penis is what counts. A woman who has sex with men is really straight, even if she also fucks women; a man who has sex with men is really gay, even if he also fucks women. If a man fucks a man, even once, he is forever corrupted from the heights of heterosexual masculinity.
If anyone of any gender has sexual thoughts or experiences about both women and men… it’s not up to the rest of us to say that they’re “really” bisexual. Or, for that matter, that they aren’t. It’s not up to the rest of us to say whether these thoughts or experiences are trivial or important. It’s up to the person thinking them and having them.