Growing bonsai is as unnatural as putting flowers in a vase, keeping plants in house, pruning hedges, roses and grapevines, cutting the grass, grafting apple trees and even collecting and planting seeds in stead of letting them grow where they fall.
Consequently, the following things are unnatural too: Having a garden, drinking wine, eating bread to name a few.
If we only did things that your cousing apparently considers natural, we would still be hunting for meat and gathering wild berries.