For those still arguing that the message is cute, or innocently funny, and that the rest of us are just misunderstanding it, please take note of this alternative design that hammers home the meaning even, um, harder, and puts it in terms you can't possibly misunderstand...
The message -- with or without any kind of graphic -- is just plain inappropriate at a place like Disney World. If my son or any of his classmates were overheard in school telling a fellow student, "Oh, look at her - she wants the 'D'!," the only 'd' they'd get would be "detention," because teens and their teachers know exactly what it means, even if their grandparents don't.
It's not that there's no place for ribald humor: there's nothing wrong with that kind of innuendo among adults in an informal setting (at your local bar and grill, or at an adult-centric vacation destination like a Sandals resort, or Vegas, as someone mentioned earlier). However, emblazoning it on your chest in front of hundreds of children at a family-friendly theme park is an unqualified reflection of poor judgment and lack of class. Unfortunately, Disney doesn't seem interested in enforcing such standards under current management, so in the meantime, we've got to learn to live with the D-bags.