Who defines the arrangement in a friends with benefits situation?

Oftentimes two people come together and decide that the time is not right for a relationship. But they may like each other enough to want to engage in a friends with benefits situation. Who has the upper hand in that situation? Do you know the rules to a situation like that? And most importantly who will Define the arrangement? You have a lot of men that will try to define the situation. But often times and Nod the rules will go in their favor. As women do you allow this or do you put your foot down and say hey we're not in a relationship I'm not giving you power? Let's talk!