Bigger Is Definitely Better (слэш)

When Derek first took off his clothes in front of Stiles, Stiles couldn't believe what he saw. Derek's dick wasn't big, it was huge. And at first he didn't care much about it, but as he started to spend more time with it, the more he sucked on it and stroked it with his long fingers, the more he fantasized about it. Suddenly he was obsessed and being fucked by Derek's gigantic cock became his #1 fantasy.
The problem was, Derek didn't seem interested in fucking Stiles at all. Whenever they had sex, Stiles always topped. It was like the possibility of Derek fucking Stiles wasn't even on the table.
Until one day it was.