>>45809552Factoring in game mechanics, the answer is Ditto. Approximating to the biology of the closest real-world organism, I feel like any and all of the plantmons with flowers are probably the most sexually active, considering what flowers are for. There may also be some invertebrate Pokemon that are based on animals that are constantly spewing out gametes into the world. And then there's Kangaskhan, who probably isn't the most active but still deserves a special prize for being born with a child already in tow. Being born pregnant isn't actually unknown in our animal kingdom (see: aphids) but it's still funny.
