Wiy: that wasn’t my main point. You made it sound as if losing weight is the most important thing, then wondering why they don’t fast.
You can date skinny girls and they can remain skinny most of the time for all I care. Just remember there’s more to life, you actually want your wife to function as well. If it’s going to be very important to you that she fast on an “optional” fast day, feel free to do all her chores that day.