The poem is written as a notice for the wall above a urinal, and it speaks in the voice of an authority that tells men how to behave. "Eyes front, soldier" sets up a military tone, with a "general" watching for any "deviation." The joke is that something as ordinary as using a public restroom comes with its own unspoken rules about where a man may look and what he may show interest in. Even the poem itself, it admits, is "probably gay," so reading it too closely becomes risky.
Then the mood shifts. The speaker reminds us that "there were trees here once," and that the man is relieving himself against a "hollow trunk." The building has replaced a living place. The last stanza, with its "saltpeter" (a substance traditionally said to dampen male desire), ends on dark humor: the trees wouldn't have known what to do with all that suppressed desire. The tone moves from mocking to quietly sad, hinting at what both nature and people lose under such rules.