Their lives were one giant ball of weird, why not throw one more turn into the mix? You knew your life was messed up and crazy when fucking your own brother wasn't the worst thing, morally speaking, you'd ever done. But damn it all if it wasn't one of the hottest things you'd ever done!
Dean's words, mixed with his grabbing hands and rocking hips was all the encouragement Sam needed. He withdrew almost completely before thrusting right back in. It only took a few thrusts for him to find a rhythm - hard, fast, desperate.