He was only returning Dean's kiss, damn it! This was Dean's fault! All of it. At least, that's how Sam was choosing to remember it and he was sticking to his story - no matter what.
Besides, it wasn't as if Sam had a vast experience with guys either. Dean was hardly soft and curvy in all the right places. He wasn't even a lanky boyish guy. He was solid muscle. Every bit not a girl.
Even so, Sam kissed him harder, using his height advantage to turn them towards the bed. He rested his hands on Dean's shoulders, giving him a push.