It's because, as the show shows the characters, they have no naughty bits when they have the chance to draw them in. To wear clothes is to imply that there is in fact something to hide from general eyes, so it's "more arousing" in this sense. On real women, there IS something to hide, but
usually it's pretty disappointingso to cover it up lets the imagination go wild on what may be under it. When characters that usually wear clothes have none on, it is more arousing for the same reason, because to have clothes is to imply that there is something to hide and when the clothes are absent, that "something to hide" is exposed, and usually that something is something to make babies with so the human mind gets excited.
Maud is for breeding.
>tfw strong believer in unicorn master race but appreciate earth pone physical constitution