Manga with a robot with a penis cannon It's about a boy and a girl, and the only scene I remember is one in which they are in a fighting robot, facing off against a bad guy in a robot with a penis cannon. The robot fires a huge shot and an old lady says "quick! After it fires a huge load it needs time to recharge" and someone else says "sadly that's true about most men".

Another line in that same scene: the girl and boy are very cramped in the robot, the boy is rubbing up against the girls back, and she asks "what is that poking me?" He says something like "it's the growing tree of the puberty forest". I don't remember the exact line, it was years ago and it made me laugh.
It may be the Neo Armstrong Cyclone Jet Armstrong Cannon from Gintama.