Some of my friends relationships are based solely on the kids (the family unit), i always worry that when those kids hit 16-18 and start having their own life, perhaps go to university etc where will it leave that couple as their way of life has gone and barring the kids they have nothing in common. One friend actually admitted he choose his wife because she would make a good home maker, not because it was his sole mate or was desperately in love, their wedding was like a business merger, very sad.
On the other hand i have friends who have a great balance, one couple i know tried for 12 years to have one, they now have a little girl and are absolutely made up.
Just remember, a kid is for life, not just for benefits