Ethan showering is the one of best parts of the game -- or at least this is how I think the game is working with the typical hardcore gamer heterosexual 18-30 male who would own a PS3 and think to purchase this game:
Are you feeling so uneasy at watching another dude shower?
Are you afraid it makes you gay, that someone will walk in on you playing and think of you "in the worst way possible?" Just keep chanting "no homo" while playing it. Then everything will be okay.
But wouldn't you expect a guy to shower at the start of the day? A lot of men shower every day -- does that fact make you uncomfortable? It's completely logical and reasonable in the sense of the narrative, that it is just another normal day.
It's the male gaze vs. the female gaze. It's mandatory because you wouldn't have made him shower otherwise.