I Don't Understand the Turn of Phrase "Piss and Moan"

I am aware that it is supposed to mean complain, or whine, or something to that effect. I just don't really understand how we are supposed to get there.

Spontaneous urination is rarely a response that I have to a displeasurable situation. And when it is, I am guaranteeably quite drunk, and on the verge of committing some form of public urination anyway. Further, moaning is only a normal response to one kind of unfavorable situation, that of bodily displeasure.

So neither is an appropriate response to a generic disappointment. I would even go on to say that if you frequently feel the need to 'moan' at the same time you do to 'piss,' you should immediately notify a doctor as well as everyone with whom you have had sexual intercourse in the last 6 months (which is to say that if you are reading this blog, you only need to notify a doctor).