Dean was pretty sure Sam was holding back his rougher side, and when another finger came he was much more comfortable with this whole idea. After all, if he was able to take Sam's fingers, he was able to take more, and all without freaking out about it being gay incest. He wasn't going to overthink it, at least for now.
The movements getting harder made his breathing shallow out and Dean slid both hands against Sam's back, fingers pressing into his flesh. Then there was that sudden shock of something and Dean choked on a strangled noise. "Holy crap... the Hell was that?" He looked down, as if there'd be some kind of explanation of it, and when there wasn't he found Sam's eyes, searching his face.
He really liked the feeling of Sam's fingers inside him. Wow, he couldn't get any gayer-- okay he could, but jeez.