What's that fan service too? How many times did we see Luffy, Zoro, or Sanji shirtless?
Man and woman are different and think different.
Its simply not possible to draw such a straight line between that tho. There is a lot inbetween actually. And Im not even only talking about transgender/transsexual/homosexual people. The patterns of what is 'male' and 'female' are highly connected to culture and different cultures have different definitions of typically male and female thoughts/behavior.
It is true tho, that fanservice exists for both sides, particularly in One Piece.
What One Piece's main problem is, is Odas inability to see women as 'normal' humans.
Theres even this old SBS for example, where he says that he picks the devil fruits for female characters based on what all women are supposedly interested in.
So Alvida got a fruit that makes her slim and her skin slippery. Think for yourself what else-beside her 'flawless' face becomes slippery...
Robin got many hands (what is that good for, thinking with the mind of a notoriously horny man...?) Also her fruit is called the flower flower fruit. As all women supposedly like flowers. Women themselves are even compared to flowers in many male-centric cultures.
Many male shonen authors seem to fail understanding what women actually ARE.
This is maybe also a side effect of most series usually being written by only one author. When many writers work on a series they can choose different writers for adding differene elements to the story that are more inclusive.