Let's say the serum causes supersoldiers to produce a huge amount of cum. Let's say Steve's been fantasizing about finishing inside someone ever since, but we're really talking enema sized loads so, that wouldn't be polite, right?
Let's say Steve and Bucky get caught by HYDRA, and they know about this and decide to have some fun with their toy soldiers.

+++ bonus points if Steve feels super guilty because he's hurting Bucky, but he also gets the biggest shameboner when they tell him to fill Bucky up