My first time seeing this post. I think it is intriguing, although we have had a suggestion for something somewhat similar before.
First of all I think a rule should be that each reward has to top the last attempt at the reward. For instance, breakfast in bed could start as just toast and jelly, but the next time would have to be better food. This would also force the couple to step out of their comfort zone sexually and try things for the sex rewards.
As far as the story is concerned, I think it needs to be about a couple who has some issues with doing chores (either one or both parties involved). Maybe neither of them had to do so growing up, so they don't have much motivation to do them as an adult. Eventually they become tired of the mess and come up with the reward system you described before.
You could also spin it as they have grown into a rut both in a relationship and sexually. They have been living like roommates for a few years and have lost the heat in the relationship. They both do their fair share of chores, so they decide to reward each other to show their appreciation and bring the heat back to their relationship.
The couple has problems in the bedroom. Great marriage otherwise. Very much in love, but hugging, kissing, and more intimate activities have gone away.
One night the person sees an ad on late night TV for a marriage counselor who specializes in sexual issues. They couple talks and makes an appointment.
The therapist talks to them and suggests a game he/she invented that will help them become more comfortable with their bodies, and intimacies.
The game consists of a deck of action cards and reward cards. Every time one partner picks an action card and completes the task, the other partner picks a reward card to show their appreciation.
Whoever complettes the most actions over the course of a month, gets a reward from the next level at the end of the month.
For example on monday he picks a level 1 action card which reads Put all your dirty clothes in the hamper for one week.
Once he's done that, she has to pick a level one reward card which might read Cook and serve your partners favorite meal to him
Now if she completes five level one actions, earning 5 points and he only completes three level one actions during the month, then he must give her a level two reward, which might be Buy your partner something they've wanted for a long time
Each month the actions and rewards increase in difficulty, expense, and eroticism. Half of the unused cards are kept each month so there are always cards from lower levels, but the scorte is based on the level. Complete a level 5 challenge get 5 points. Complete a level 1 task get 1 point.
The cards can be customized for the couples, and there are generic cards either can use and there are some just for girls and some just for guys.
COuld even be a multo author series, where each other creates a couple who uses the game to liven up their marriage...