Art comes in all forms. You can look at a Rembrandt painting and say his mastery of shadows was the antithesis of the Baroque movement that characterized much of 17th-century Europe. You might scoff at a contemporary art piece that, to you, looks like a coffee spill on some printer paper but, according to the artist, "like, totally captures the spirit of America and stuff."
While we can all objectively say that the coffee-stained paper isn't going to be studied by scholars hundreds of years from now, both of these examples are, technically, art. That's because art isn't defined by its quality but rather by the expression of the artist. To quote the American poet Muriel Rukeyser,
"a work of art is one through which the consciousness of the artist is able to give its emotion to anyone who is prepared to receive them. There is no such thing as bad art."
In some senses, Leonardo da Vinci's anatomically correct Vitruvian Man and that giant wang that some infantryman drew in the porta-john in Iraq are more similar than you realize. Not only is a penis central to content of both works — both also fall in line with a given art movement.
Art movements generally follow a few guidelines — and "grunt graffiti" fits within those. The artists (troops) share a similar ideal (discontent with a deployed environment) and employ the same style (crude and hastily drawn) with the same technical approach (permanent markers on walls) to create art within the same time frame in a similar location (Global War on Terrorism).
The general public mislabels the simplicity and minimalism of grunt graffiti as being "unengaging." But Pablo Picasso is also often placed in this category, too, despite his skill. In December 1945, he created a series of 11 lithographs that began with several masterful sketches of a bull. The lithographs, in sequence, became increasingly abstract while still preserving the "spirit" of the bull — a slap in the face to those who confuse proficiency and artistry.
Grunt graffiti is so entwined with military culture that you can find it in almost any stall. Some are elaborately crafted and some are simple doodles. Some are drawn out of boredom and some are made to tell the unit how that troop feels.
Granted, "grunt graffiti" is, more often than not, some kinda crudely drawn dick. Now, we know that nobody is actually going to examine these porta-john decorations closely (unless it's to punish the artist for vandalism), but we maintain that if a canvas painted white (known to some as "monochromatic art") can sell for $20 million at auction, then recreating the frescos that adorn the Sistine Chapel (with all prominent themes replaced, primarily, with dicks) should at least get a little respect.