I agree that it would be nice to have your own children! I myself would probably have preferred that option if I were presented with it as a player. In the perfect scenario, we would have that as an option, but there are several reasons why we opted for adoption only:
1: Having your own children would mean more graphical assets, which means more work for an already very large game. It would also require checking your player character's appearance against the appearance of your spouse and so on, which would make things quite complicated to implement
2: Having your own children would mean that the player would need to have an assigned gender, which we want to avoid. Alternatively, it would mean there would have to be some sort of "magical" childbirth (a stork or something), so that the gender of your character and your partner don't matter, which I personally am not a fan of
3: There are some characters that you wouldn't be able to have your own biological children with, which means that adoption would still be necessary in order to have a child with all possible spouses (if we assume there is no "magical" childbirth, of course)
4: By only having adoption, we can ensure that the children you can have are actual characters. Otherwise, they would likely be assigned one or two character traits, making them feel like cardboard cutouts and thus be almost impossible to give unique and interesting stories - especially endings. It would also make them a lot more complicated to implement. With adoption, we can also make the children a bit older, so that they can actually display their personalities, and not be static babies in a crib. The biggest reason why we went for adoption is to avoid the feeling of the children being props, which they often feel like in other games (like Stardew Valley, Story of Seasons/Harvest Moon, and probably Fable, too. I haven't played any Fable-games, so I can't say for sure)
I hope that answers your question without making you too disappointed! Hopefully, our decisions make sense when the game is fully released and you'll get access to the adoption-feature and see what colorful little characters the children are ;)