In the first generation of pokemon, R/B/Y, pokemon had no genders. That feature was added in Generation II. So if they wanted to have pokemon of a specific gender, they would have to make separate pokemon, like with Nidoking and Nidoqueeen.
They figured you could just guess as to what gender the pokemon would be. More likely than not, Jynx would be a female, and as is the Case with Mr. Mime, he would be considered male. Though honestly, I think they should have made them one gender only pokemon, like Hitmonlee and Hitmonhan.