Look.
I get arguing about shit to argue about it, its a past time of mine. In this instance I think the issue becomes the reason its bad is something hard to articulate because it gets into big social theory shit. When the family unit becomes one in which romance is allowed, it starts to break down the family unit because it ceases being adults protecting children and becomes a grooming/exploitative situation or one in which a non-desirable child gets kicked out of their little pack. From there, the break down of a family unit ultimately will reflect in the ties between non-family members in society so it does all start to impact people outside of the two involved in the sexing.