Starfleet is only a military organization when the mission requires it. And from what I know of my military friends, yes, acting like a juvenile idiot is sometimes exactly what happens. It's not like they do it while they're in the middle of a crisis. Voyager isn't just their place of work; it's their home.
I run a pizza shop (its fun being the Janeway of something) and it's exaclty the same here. We goof off and have fun from time to time (we call it fucking around, but no sex is involved) but they still stand and salute when I walk through the door.
Now Paris, he is my kind of guy. And he scored the hot Klingon bird. +1 so to speak