Does arousal for the slightest reason, such as looking in the mirror and contemplating the manifestations of masculinity, have any religious rulings, or is it considered insignificant?

If looking at permissible sights arouses desire and one takes pleasure in it, then intentionally looking at them is not allowed. Some jurists have explicitly prohibited looking at what is permissible with the intention of lust. It is also disliked for a person to look at their own 'awrah without need.