lavendersparkle

I'd assume the major response would be, "incest hurts potential children". As rjw76 commented, this doesn't apply to couples who can't have children. Various surveys claim to have shown that most incestuous relationships are abusive. Whilst this will be heavily skewed by the fact that abusive ones are more often brought to light, protection of children too young/too obedient to parent to say no would probably be another major argument in favor of incest being classed as wrong.
~KT