She looked like she was a teenager, and if we factor in how old Twilight looked, Cadence might not have even been a teenager when Twilight was conceived.
So that's three major flaws in your fanon:
1. Rather than explain something that wasn't explain, your theory completely goes against what the staff's intent was.
2. It involves characters acting in convoluted ways, and I don't just mean Cadence and Shining Armor marrying immediately or after Twilight was born. I mean they're just now
marrying. Cadence could've married years ago.
3. The implications of this theory are just disgusting, and you should feel ashamed of yourself.