I know it seems confusing, I'm not the best at explaining my own thoughts. What I mean is that, in general, "porch monkey" is a term that would be used IN a phrase, not a phrase in and of itself.
And I'm not saying that just because it has only two words in it. A phrase can even be just a single word, according to grammar websites.
In fact, I'll grant that if you add an S on the end, "porch monkeys", it could be a phrase, much like women say "men!" or men say "women!", because there's an implied but unspoken "fill in the blank" continuation to the thought, like "Women! Can't live with 'em, can't live without 'em."
It may seem like splitting hairs, but I can't really think of a way to explain my reasoning any better.