I'm implying that two people who perform incest have at least one child, yes. If they don't have a child, then my argument against consensual incest is moot. However, I'd rather prevent consensual incest all the same to decrease the likelihood that recessive genes that malform, or otherwise disable, a child are passed to their potential offspring. Much like I'd rather prevent my own child from even touching a knife even though he might not get hurt if he does touch it. Prevention is better than cure, as they say.