Since when do the sages use bathroom humor?

This week's parasha teaches about the mann (manna) that God sent to feed us in the desert as we wandered after y'tziat mitzrayim (the exodus from Egypt).  B'nai Yisrael (the people of Israel) were complaining tremendously about the lack of variety of food in the desert.  They even started to yearn for the food in Egypt!  This made God very angry after all S/He had done for us.  Even when God sent the mann, we were not satisfied with this blessing.  The complaints were numerous.  There was not enough variety, or there was a fear that the mann would run out and they would try to collect more than a day's worth at one time.  In this midrash, chazal (the sages) bring a particularly funny example of what b'nai Yisrael complained about
When God brought down the manna for them He wrought for them many miracles. Our Rabbis said that they had no need to ease themselves as is otherwise usual with mortals. Why so? For no other reason than this: God said: ' If My children are to have need of easing themselves, how can I cause it to be written concerning them that they are god-like beings, as I did in the text, I said: "You are godlike beings." (Ps. LXXXII, 6)? This must not be; but even as the angels do not need this thing, so shall they also not need it any more.’  How did they respond to the favor which God bestowed upon them? They began to say sneering things about the manna. One said to the other: ' Do you hear, brother Simeon?’ The other said: ‘What do you say, brother Reuben? ' Says the first: ‘On your life! Have you ever seen in all your days that a man should put wheat into the mill and that this wheat should not, after being crushed, run down? We, however, eat the manna and it does not run down!’ His companion responds: ‘I fear that ultimately my stomach will swell and burst, since we cannot excrete it.’ At that instant God said to Moses: ‘I can no longer suffer and control My anger, "How long will this people despise Me? and how long will they not believe in Me, for all the signs which I have wrought within them?" (Num. XIV, 11) Midrash Rabbah - Numbers VII:4
To summarize this midrash, b'nai yisrael was compared to angels in the book of Psalms, and called "God-like beings."  Although being "God-like" can mean many things to many people, apparently to chazal one of the God-like attributes is not needing to use the bathroom.  It might not have been one that I would have thought of, but then again, I am not one of the sages.  In any case, they read this pasuk (verse) and then begin to wonder when it ever actually happened that  b'nai adam (people) did not need to use the toilet.  They arrive at the conclusion that this happened during the time of the mann.  

The wisdom of the midrash and its meaning for me is most powerful in the way we react to the "miracle" of not needing to use the bathroom.  After all, it would be alarming to most people if their bodies suddenly started functioning differently.  But the reality is that it was a blessing that our bodies did not work the typical way.  It made us like angels!  This can be difficult to discern, much as many blessings are difficult to discern.  Sometimes God gives us a blessing, and we cannot recognize it.  We might even fear it!  The lesson is to stop rationalizing and analyzing and simply experience the blessing.