It's facetious to say "Oh, the protagonist has seen all the girls naked, so he should be fine", because the motivation for having the dryads wear clothes isn't purely selfish in nature. The vast majority of the population of the village are uncomfortable with the dryads being naked in public, so he's asking them to wear clothes. The dryads don't seem incredibly averse to wearing clothes, they just seem to find the practice kind of odd.