Well, I did say it would depend on the character! But it's pretty neutral to me (there's actually a discussion about this exact word going on in a comm I follow, too! Coincidence?) and I'm sure some characters would find it neutral, too. I could certainly see a geeky virgin, or a Vulcan, or an English major, for example, noting the spongy texture in those words and finding it excitingly sexy/new/alien/evocative.

"Girthy" is one of those Nifty.org sorts of words, for the most part. :D I rarely encounter it in fanfic, but if I did I would probably just guess that the author has read some original web erotica and move on without a fuss.