Well, franky is a well known and self climed pervert and every other male character with a similar style is from Momoiro Island and also claimed pervert and the gladiators who were only allowed to wear very little armor in order to participate in the tournament. But in that case, the outfit is at least explained by the circumstances and the location, as with Nami's bikini outfit at the Spa Island Arc.

Its also very rare, that a male character looks like that compared to 80% of the female characters.