Your symptoms are suggestive of prolonged sexual suppression.
Everyone past a certain age will have come across a boner, whether you have them, see them in movies, or have had any sort of sexual experience with a guy.
Like seeing you naked when you two are in the middle of a fight—yes, most men are just that visually oriented.
What Makes a Penis Erect? And it'll go away eventually.
Who is the most phallic superhero of all time? Just let it pass, don't make a taboo out of it, but don't draw attention to it, either.
But for most men, those things can pop up at pretty much any time for any reason.