Diana not having a thought about a kid is probably due to the writers. There is stigma associated with superheros as parents. It 'ages' them according to many. Her neglect has been brought up in another thread. There were imaginary stories with kids for Superman or Batman. People find it somewhat difficult to think of a mother and a superhero. I am not painting everyone with the same brush. For a lot of them its more subtle. Something even writers may not be aware of.
I pin the in story reason as Diana being busy. But since you have brought it up it does seem odd. She is best friends with Bruce and Clark. It should have crossed her mind.