tbh, childfree characters are realistic in more ways than typical fanfic tropes are. Most characters in shows (the ones I watch) do not have children, so who is to say what their stance on having children is? Making them all childfree is way less forcefully bending them than "making everyone gay" or "making everyone a superhero" or "alpha/beta/omega". All of these tropes are incredibly wide-spread.

People complain about them, too, but they have just as much right to exist and be explored. Fiction is about exploring interesting concepts. That is always reason enough to do it.