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the opponent could very well switch, and assuming a standard team with gendered mons, the Pokémon you use the move on has a roughly 1/2 chance to be male or female.

Also, using 13% genderless Pokemon doesn't really work either, that kinda supposes that every Pokemon is used equally. But really genderless Pokemon, since legendaries tend to be genderless, show up a lot more often than some other Pokemon. You'd have to break down the actual usage stats if you wanted to use that as a calculation.