Sam asked Dean where he wanted to do it and he chose to stay in the shower. Once again it was all Dean's fault. Even if the whole sex in the shower thing made Sam feel a heck of a lot younger than he was. Randy teenagers and people in their early twenties had sex in the shower. And why did Sam have to make all the hard decisions? Damn.
"Well, let's see... facing me means I can watch your face as you come and I can kiss you whenever I like..." Sam trailed fresh kisses along Dean's neck. "Or I could pin you against the wall, rough, hard, and take you from behind... Your choice, big brother." Dean wasn't the only one with a velvet voice.
And Sam had totally put the decision back in Dean's hands. Smooth.