Ludwig riding Feliciano hard, totally needy and submissive and embarrassed, and probably moaning like the whore he secretly enjoys being.

Bonus: Feli is very much more experienced, or at least less sexually repressed compared to Ludwig, and goes all smirky and 'I like it when you make noises, ludi~!'
Bonus 2: the actual 'save a stallion' line is said at some point and causes massive facepalming because Feli is a dork

If you fill, I will love you forever and probably have your babies