How is consensual incest harmful?

and don't bring up deformed/retarded babies or future regret, okay? my parents are twins, and they had me, my 2 sisters, and my older brother, and none of us are abnormal in any way. and they had no regrets of what they did. so answer this: how is NON-reproductive, mutually consensual incest, without any regret, harmful to the individuals participating in it?