I think I've been perusing TVTropes for too long. Like this little nugget:
And the real kicker - who's to say it doesn't do this to humans?Ditto can breed with any other Pokémon, which makes perfect sense since it can Transform. But really think about what that means. It is Transforming into other Pokémon to induce them into having sex with it (or however it is that Pokémon breed). And of course when you leave Pokémon at a day care center you don't actually tell them the purpose of such is to get them to breed, so Ditto is doing this of its own will. Ditto is an in-universe Memetic Sex God/Memetic Molester, able to seduce any (non-gender neutral) Pokémon by shapeshifting into their image of a physically ideal mate.