This request can sound silly to some people, but it's really a great idea for older couples that have gotten out of the habit. Sex, for a married couple, is emotionally intimate and is an extension of the trust and happiness that the two people find in each other, among other things. (I understand that the sex that goes on at college drinking parties is quite different, but we're not talking about that, today.) In order for a married couple to have sex for seven days on end, they must not go to bed angry with one another for that long. If the two are lecturing each other about what they did wrong that day or harboring some unspoken bitterness, you can bet that their evenings will be uneventful.
It isn't completely about the sex. (But, it is mostly about the sex.) In general, a married couple that has regular sex is getting along well, thus the couple is happy and the home is more stable, which is good for the children. (Who are bound to come along if this trend keeps up.)
I'm glad that this sort of thing has caught on in churches and I hope that married couples everywhere can learn to benefit from it. (Here's more on the idea behind this challenge, if you are interested.)