God is The Trickster's (archangel Gabriel's) father ! ...
Many characters/scripts are developed in the series in such a way as to show how one matter or main character affects another.
(Maybe needless example. When we see the interaction between God/Michael and Lucifer with their respective vessels/swords in the early show it intends to show how important Sam and Dean can get to be and how they are considered in the big picture. So S & D are important because of their past, how they were raised, and so on. Likewise, some personality traits are present too when Metatron explains his, many times passive, role through all times. However he chooses something different when needed.)
God being Trickster's father may convey the idea that all his creations have some relationship to his own personality. Through season 11 He shows how He enjoys doing several human like activities such as eating bacon and watching tv, these characteristics being pretty much alike to that of Gabriel's. That could even consider types of porn, since Gabriel also was a bit of a fan.
Of course politeness (good point on another posted answer) can play a role in the God-Human interactions, but this alone may not be enough.
As an end note: perhaps He has some pleasure in checking people's reactions, just like the Trickster. He could just as well say:
- Hey guys I feel like taking a shower are you all cool?
Same thing but with no need to ask for the place where it is. As the conversation actually went, it could have been answered with some slight giggles, and
- Sure you can go, you know where it is.
Also, on the following episode, God says that He has some guilty pleasures. Although not sure if this may, or may not, be enough to explain why he watched xxx ...