Please, PLEASE, don't even try to put DaVinci and that sticker on the same level. And I doubt that the nude art nearby is in the same class as the sticker either. I would be irritated to find it, just as I'm irritated to have to look at t-shirts of the same category as the sticker when I walk downtown or through the mall. Fair warning seems appropriate, at the least. I feel sorry for people who look up with excitement to see the hot air balloon, and have to see a giant picture like that. Whether or not it would be classed as "porn" I won't argue, but it's obviously meant to "titillate" (how do you like that word?).
P.S., I hope I never find a cache containing condoms and lube. I have found caches with such as litter nearby, which always causes me to say "ew". I don't care for close encounters of the body fluids kind. I recently read a log for a cache in a public park that said the litter nearby included a hypodermic needle. I appreciated the warning, especially since I would have worn flipflops had I looked for it.