Well, first I like both
I do agree that the characters in Naruto are attractive
but I feel the reason that the characters in One Piece are seldom seen as 'hot' is because of the focus of the show.
Naruto takes place with a group of adolescents who are essentially going through puberty who become more serious
One Piece on the other hand starts with people post-puberty who have very tragic backgrounds and who are almost always very isolated from [at least] mainstream society.
They're both great anime. But Naruto copes the darkness of the story with 'potential bachelors and bachelorettes' and scattered humor.
One piece only addresses sexual tension when it is embedded in a characters personality (Sanji and Hancock), and the importance is more about how other characters react to these aspects, not the viewer. (which can motivate authors to do shit like spin-off series, ugh)
Instead they break the tension with comedy instead, and keep the focus on action-drama, and not potential-romance-drama.
To sum it up, Oda is focused on doing the story right, not pandering to fan-service
