[ikeyany]

Thought experiment time (perhaps this happened to you in real life as well): suppose you saw this attractive female body from behind. Then as she turned around, you noticed she was a man. Doesn't that mean you are also attracted to men in some way? At that point it's no longer binary—it's just a matter of degree.

[growlist]

I'd say that proves the opposite - the moment of realisation is when your brain identifies the sex of the person, and then discounts them as not being of interest.

[ikeyany]

Not being of interest yes, but it doesn't take away the fact that you found them sexually attractive in some respect.

[WalterSear]

Before they turned around, you had a mental image that filled in the rest of the picture.