I'd better add my perspective.
I think a bull is someone virile, competent and confident in their sexual ability who is equipped to fill a physical and psychological desire. They should have a respect for the relationship and recognise their place in it. There's kind of an obligation to fulfill the role, to be clean and in decent shape. I think a bull should be aware of what the couple need, but also have a degree of selfishness, taking what he wants, meaning that he isn't just providing a service, he's there for his own needs too.