Here's my first thought - the natural state of one's soul is to be whole. A broken soul, a soul split into pieces, is no longer natural, no longer whole. This strikes me as a ... bad thing, and hence why doing something bad, something evil, does such a thing to a person's soul. Killing in unnatural, and does unnatural things to the killer - rips their soul in two. This is not desirable at all.
So I think that an act of love, of creating a child - something so natural - shouldn't be something that splits the soul and creates something unnatural. It just doesn't seem right, it seems the opposite of what nature intended. Does that make sense? A Horcrux is Dark magic - birth doesn't seem the way to bring about something so terrible.