I don't think it's so much disappearing (I seem to have one, scratch the fence and with a kitty instead of a dog) as it is that alternate family situations (mixed families [families with step-parents and/or step-kids], single parents, gay couples as parents, etc) are becoming much more common and socially accepted, which they certainly wouldn't have been only 30, 20, or maybe even 10 years ago.
"Wanting people to listen, you can't just tap them on the shoulder anymore. You have to hit them with a sledgehammer, and then you'll notice you've got their strict attention." - John Doe, Seven
Heather Rose Reptiles