I think that when the 'ideal nuclear family' portrait was painted it was in an era when it was important for husbands and wives to stay together for 'the sake of the children'. Now that women are more independant, and divorce has increased significantly over the last few decades, that yes, it is disappearing.
Now look at 'modern nuclear families';
With Homosexual persons starting families, there almost certainly are many two-parent households raising children. The only difference being, of course, both parents are of the same gender.
Children Being raised by relatives, as opposed to being raised by parents.
These are just a few examples of "Modern Nuclear Families".
I have to get back to work... I'll post more on this soon...