If incest is made legal then it would be difficult to regulate because obviously it is not acceptable to allow such couples to have children. That would be a heinous thing and I don't think I should need to explain why. So that's one very good reason to outlaw it.
Another reason is that if incest is made legal lots of young children will be growing up seeing it as perfectly acceptable and inevitably lots of sexual relations will start arising within families. You could say now 'but the idea of ****ing my mother is disgusting!'. Why? Although there is some nature involved, society's labeling of it as wrong has partly made you feel that way. Start encouraging it and lots of confused children will become sexually attached to their siblings/parents, whatever. That would be a bloody terrible thing too. We need family, and a strong family bond is impossible when sex is involved.
There are plenty more reasons out there but those are two good ones, even by themselves.