They have sex in the shower a lot less frequently than Dean is sure Sam assumes.
It isn't just because the shower is too narrow and slippery to get the right angles - never underestimate the determination of two people in a new relationship who can't keep their hands off one another - or because the capacity of the water heater leaves something to be desired. Nor is it because time and familiarity have put a damper on their ardor.
No. Often, it is a pleasure in itself to watch the water run in rivulets down Cas's back, to reach out and dig his thumbs into that spot between Cas's shoulder blades that makes Cas's head loll forward in something close to bliss. Cas's shoulders are always tight, as though the knots have knots - Cas has told him it's because it's still habit to hold himself as though he has wings - and the combination of the hot water and Dean's hands loosen those knots, even if only a little bit and for a little while.
And then, of course, sometimes Cas will take a palmful of shampoo and run his fingers through Dean's hair, rubbing Dean's scalp with a purposeful concentration that never fails to send warm, lazy thrills through Dean's body. And he keeps massaging as he pulls Dean under the showerhead to rinse, and it makes Dean feel strangely purified by the end of it.
The glowing feeling of being clean is a source of contentment, and as they get dressed, they're usually distracted by warm, shower-soft skin and tousled damp hair and -
They frequently take two showers in the morning.