You just need to defeat them via seduction (5 or more CHAR reccomended.) Bull warriors usually stick mostly to either fighting or seducing, but if you flirt with them, theres a chance that they might flirt back.

At that point it's just a battle of who seduces who first (which is why I recommended 5 or more CHAR, cause their flirting moves fill up your lust meter pretty quickly.) 

If the bull happens to be stubborn and confrontational (attacks a lot and ignores some of your flirting), have some potions and enough stamina for "Bind Up" ready, because those guys hit pretty hard in early-game.