Why are so many mangaka perverts?
I assume a lot of mangaka grew up admiring the works of other writers, which probably means reading a lot of other manga or watching anime.
A lot of works which feature more perverted content in the industry.
Oda was probably also part of the generation where it was super common to see such content.
Dragonball had a lot of weird stuff when it began, especially in regards to Bulma. Doraemon was also a thing.
We may have the entire ecchi stuff these days but back then, it was even normalized that 12 year old Goku had his penis out.
And that is Toriyama and his influence on the industry, I doubt I have to explain.
At that time, manga went through a spiral of evolving in content.
1981 had Brygar for instance which was the first sex scene ever in anime and that was censored but then you have Dragonball and Doraemon. I suppose you can add Ranma too and even Sailor Moon exists where Takeuchi had official art where the characters had exposed breasts.
Oda grew up with this, as did many others.