When unmarried couples date, they should be forced to rent a child for the duration of the evening (some call this "Markets in Everything"). That way they know how well they do when there is a child around. Special bonus deals will include children with runny noses and twins who slug each other. Note that this also will improve the marriages of the people who are renting out the kids.
Once a couple have their own, they can dispense with the rentals. In the meantime, they are building their relationship around the presence of a child. They know what to expect.
Alternatively, one might vow to only date people who already have kids.