Yes, because if they were born the opposite sex then who knows in what fundamental ways they still are the opposite sex. Not being able to tell doesn't mean everything. The only way that it really wouldn't matter is if you completely rewrote their identity from the atomic level (and, to be truthful, probably their spirit too), in which case you might as well have destroyed them and made another human in their place. And even if you did that I don't trust a human that was completely invented by technology as far as their dateability goes.