Maybe a little bit of both. He'd learned to enjoy a lot of strange things over the years. As for the whole 'brother' thing, they were definitely masters at ignoring the elephant in the room, and Dean was already subconsciously going over the plan for his meltdown after this. The problem was actually the fact that he enjoyed it all too much to think about much else.
Watching Sam, one arm tucked under his head, Dean buried a hand in his brother's hair, and he couldn't help petting him in an encouraging way, like he would occasionally do when Sam was very young. "Nghh-- feels good, you're doin' good," he assured him, trying to ignore the fact that Sam was the one sucking his cock.