What does “it’s not going to suck itself ” mean?

In Goa, I see tourists wearing t-shirts that have the following text on them, along with a red arrow:

It’s not going to suck itself

What does this mean?

Matthew has half the answer. The other half is that "{Noun} isn't going to {verb} itself" is a common phrase implying that a task isn't going to get done unless someone (by implication, the person being spoken to) quits messing around and starts doing it. For example: "These dirty dishes aren't going to clean themselves."

So basically this shirt is supposed to be a "clever" way of suggesting that you should drop whatever you are doing and give the T-shirt wearer oral sex. I'm probably no expert, but I'm guessing he doesn't get many takers...

It refers to the genitalia of the (presumably male) person wearing the shirt.

(I hate offensive tourists.)

These t-shirts are apparently not limited to Goa, but appear to be rather distressingly popular, coming not merely in t-shirts alone:

but also in many other formats, only a few of which are routinely hidden from the public eye:

All these products are saying the same thing: the “it” refers here to a man’s private tackle, so this a man on the lookout for a new fellationship ^{(Urban Dictionary)} — that is, he’s looking for a blow job from someone else. The phrase “doesn’t suck itself ” further rules out autofellatio ^(Wikipedia). It’s extremely coarse advertising in search of a fellator or fellatrix to sexually service the fellow.

A slightly less gender-biased variant is occasionally seen, one that might be worn by either men or women because it substitutes lick for suck:

Even if the allegorical ice cream cone on the lady’s t-shirt might bring a smile to some viewers, all these products are extremely coarse, and probably offensive to many viewers.