what do you think about sexualization of pokemon species. Some historical antecedents?

I’m afraid I don’t really know what you mean by “historical antecedents” in this context… do you mean like bestiality in mythology?  I’m not by any means well-informed about world mythology generally; I would only consider myself an expert on Greco-Roman myth, and I think what’s going on there is a very different sort of phenomenon.  When Pokémon are portrayed in a sexualised manner in fan art and the like, that tends to involve accentuating their human-like traits, particularly feminine ones.  The most important cases in Greek mythology involve a male god in the form of an animal (or in the case of Pasiphaë a male animal and a human woman under the power of a god), and make no effort to humanise the animal form in any way.  I think the point there is probably something about humans being at the mercy of nature and the divine, ’cause sex is almost always about dominance in Greek culture, and the exemption of deities from human rules and social norms.  

A different sort of case again, where a non-human thing does have its human traits emphasised and sexualised, would be creatures like mermaids, and in folklore those tend to be seen as devilish temptresses who are out to kill men, like rusalkas in Slavic myth, so those are about the dangers of temptation and, essentially, a fear of female sexuality.  With Pokémon the human is imagined as being emphatically in charge, and the Pokémon are probably in a position where they will habitually seek their trainers’ approval… which makes the whole thing a bit icky the way I see it, in the same way as sexual relationships with children (particularly between teachers and students) are icky.