Prescripted 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.
To prescript a relationship, then, is to 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, such as a married couple deciding the rules for a future extramarital partner before even finding one.
By contrast, a descriptive relationship describes the relationship as it is at the time with input from all involved.
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. They prescripted their relationship.