The point is not to be a gender detector. The point is to be a vague heuristic to discriminate by. It's like ad network tracking. They really don't know me as well as they think they do and pretend they do, but if they can convince themselves and their customers, it's enough. If computer gets your gender wrong, well nobody's perfect and it's a sacrifice they're willing to let you make. If the computer gets your gender wrong because you're queer, gender nonconforming or a person of colour, all the better — that's what the customers want anyway.