Prescriptive comes from Prescribe: Latin, praescribere =
1. to write at the beginning, dictate, order;
2. to lay down a rule : DICTATE;
3. to lay down as a guide, direction, or rule of action : ORDAIN;
4. to specify with authority.
A prescriptive relationship, then, is a relationship in which a person or people create a set of rules governing the relationship before the people involved ever get into said relationship. These rules are often set up to define a relationship involving someone who is not even in existence yet and therefore cannot give his or her input into the relationship agreements.
By contrast, a descriptive relationship describes the relationship as it is at the time with input from all involved.
John and Mary have a prescriptive relationship. Even though John and Mary haven't met anyone they're interested in yet, they have decided beforehand that anyone they date will be female, in love with them both equally, single, will live no more than 10 miles away, will not call John by the nickname "bunnylips", will not eat with them at their favorite restaurant, and cannot ever listen to the song John and Mary danced to at their wedding while John and/or Mary are in the room with her. When they meet her, these rules are non-negotiable and she has no say in them.
The slang term for a chain of fast-food restaurants called Chik-Fil-A. Usually used in a derogatory term to refer to their strongly Christian policies (being closed on Sundays explicitly for their employees to attend church), anti-gay policies and extremely large financial contributions to anti-gay-rights bills and amendments, both of which stem from the owners' interpretations of their denomination of the Christian bible.
I'm hungry, wanna go get some Jesus Chicken?