I and others in the family or extended family, we were all taught not to have children before marriage. (I don't have children). If someone did, people would talk and talk (criticize) about that person and never hear the end of it. Back then, I believed in it cause this...
but I do believe that marriage is the best possible environment to raise children.
Beyond that I also consider it very sinful to have sex outside of marriage anyway, regardless of having an intention to conceive a child or not.
For a few important reasons:
It promotes poverty, based on observation...maybe because of the single income thing
It is a sign that someone was irresponsible, or immature, or deceitful, or not very smart; and that they decided that would be a great way to start off someone else...