I can't seem to find this fic again. It was hilarious and sexy at the same time, a hard feat to pull off. Here are the details I remember:

1. Sam/Dean Wincest, first-time
2. They get hit with some sort of pollen and have a short amount of time to have sex, or they will die
3. Dean refuses to have sex with Sam and drives around until he finds a horse (or maybe a cow) in a field and attempts to have sex with it (rather than Sam), unsuccessfully.
4. Sam/Dean end up having sex in the car (they both top at one point)

Thanks!

Story found, see comments.