Holden is incredibly bothered by the "fuck you" signs he sees on walls. He's even more bothered by the locations in which he finds them – on the wall at Phoebe's school and in the once-sacred tomb in the museum. These are places that remind him of his own childhood, that would made him feel secure and comfortable and – dare we say it – happy? Yet in Holden's world, this isn't possible; everything has been corrupted by vulgarities. Even his own death, he says, couldn't be sacred or peaceful, since someone would probably write a "fuck you" on his very tombstone.
We argue in Holden's "Character Analysis" that it's no wonder that he sees sex as dirty and degrading, when all the portrayals he's seen of sex are in fact dirty and degrading. This is one such portrayal; Holden imagines a "perverty bum" sneaking into Phoebe's school at night to write the message on the wall.
The most interesting thing is that it's more likely that a student at the school vandalized the wall. Holden, of course, either can't or chooses not to see this; in his world, children are innocent and adults corrupt. While this keeps everything nice and simple in Holden's mind, it also makes it impossible for him to really understand sexuality and the process of growing-up. While he does come to the realization that trying to rub out all the "fuck you"s in the world would be an impossible task (as there are just too many), he doesn't make the important connection that it would be futile anyway. Children will grow up and experience sex, whether he likes it or not.