It is, as there are definitely things you can infer about reality from it. In this case, ideas espoused on places like reddit are indicative of how certain sets of people (e.g left wing liberal millenials, who enjoy their computer time, and aren’t religious) feel towards children.

The things to bare in mind (which you hint at) are just: A) no matter how many people you see on the internet, it’s still a very very small fraction of real people in ththe world, or even in your own nation. And B) biased sampling - specific groups of people flock to certain sites, feel like eharing their views on the matter, feel strongly enough to share their views on it over and over again.