Just because someone might put on the "serious face" doesn't mean they are, or their arguments or comments are any more thought through or serious. Good humour is always serious. I think that people who can't laugh at anything are a little dead inside.
Humour can be great at opening up emotional floodgates. A friends grandmother had died a while back and when he told us he looked devastated but put on a face. Then another friend burst out with, "bing bong the wicked witch is dead" after which it was dumbfounded silence for a while while my first friends face went through every conceivable emotion. Then he collapsed on the floor bursting out in tears, which was exactly what he needed.
Humour is great. It's our most powerful tool to bond and most powerful weapon to break down barriers. I think it definitely has it's place here and in bed/dungeon. Whether or not you want it in a scene depends on what turns you on/off, off-course. I have no problems with it. I don't take myself very seriously.