Given how poorly most men understand women and their sexuality, it's a wonder females aren't much more often homosexual.
As for myself, and I think this is true for how it works in males, orientation is very 'fixed'. A very limited range of stimulation is erotic. Knot holes don't 'do it' for me, animals don't, machines don't, children don't. Women do. Not every woman, but when it happens, it's centered on female humans. Males are just not erotic to me. Because women can be erotic, I can imagine that for some men other men might be; I just don't have that attraction, though there have been men I loved, starting with my father. Love doesn't necessarily have anything to do with eroticism.
I think female sexuality may be less focused and exclusive than male, as is true in some other aspects of gender differences (sports, violence, politics), so perhaps it is easier to 'drift' between sexes without having to decide one is all one or all the other. But, that is merely an impression, so don't try to build a case that this post is maitianing it's true.