My guess is that the message on your T-shirt functions in several ways. First, as a bold and plain statement that you are confident about your desires, confident enough to have them spelled right out across your chest (nice chest, by the way). Second, it implies that you are a man of action, perhaps a little impatient, but a man able to state exactly what he wants. I think that's a great quality. Very admirable. No dilly-dallying with you. Third, it is possible that the T-shirt is a protest against the design of the human body, a comment about the limitations of our physical being. Did you know, for example, that if we had one set of ribs less we would all be able to bend over and do the job for ourselves? I like to be reminded of that, and I love the idea that if it were somehow useful for survival, and presupposing that Darwin got most of his shit right, we might someday evolve to where we all have penises that could actually suck themselves. Wow.
On its own, your T-shirt hints at a fairly crappy sense of humor, unless, of course, you are wearing the message for a reason. You could be promoting health and safety in some way. Perhaps you are an aid worker from Borneo or Australia, where it would be important to know that a bite from a poisonous snake or insect isn't going to suck itself but is going to require immediate medical attention. In which case I'd have the information in at least one other language. It's also incomplete, and might inspire the wrong response. I think you will find that most doctors do not recommend that venom be sucked out of a bite but, rather, advise that a tourniquet be tightly tied above the bite or wound—in which case the text should read "It Isn't Going to Suck Itself and Neither Should You!" I would also suggest a diagram of a little snake or something, just to make it perfectly clear.
Alternatively, it is possible that you are a sex therapist who works with people who are so anatomically naive and sexually inexperienced that they don't actually know that a penis can't suck itself. Hell, for the longest time I believed that women shat out babies, that babies came out of their asses! I really did! However, as an adult, I take it for granted that my sexual partners will have a better understanding of these kinds of things, but it's good to be reminded that we shouldn't always make such assumptions. In which case printing it on a T-shirt is a very, very good idea, and saves on unnecessary explanations and conversation—provided our prospective sexual partners can read.
To close, a small complaint. After you got off the Underground, the other passengers had to sit between stations listening to a small child ask his father, "What can't suck itself, Dad? Dad? What can't suck?"