Prenuptial agreements mostly deal with what happens in the event of death or divorce. But they can also be used to set up how a couple will handle their finances during the marriage.
It is also possible to put non-financial provisions, so called lifestyle clauses, in a prenuptial agreement. So a couple can agree on who does which household chores, or how much television watching is permitted or even how may times a week you will have sex. You can set a weight limit. You can agree to a penalty for cheating.
Some couples view these as valuable provisions, sort of like a mission statement for their marriage or an extension of their vows. Others find them to be negative saying that a marriage with lifestyle clauses is doomed from the beginning.
How they would be enforced is another question. Will the court order an overweight spouse to lose 10 pounds?