Morally and Ethically There are tons of arguments showing how incest - even consensual incest - can permanently effect a family unit in a negative way. However, what people often fail to see in such circumstances is the moral and ethical implications.
Of course, for those who follow a code of moral absolutes, this argument is hardly even a consideration. Sex is an enjoyment and gift to be experienced between two people who have comitted to spending the rest of their life with each other. It is like the icing on the cake of commitment.
That's great and understandable, but why should everyone be forced to follow this standard? I believe that we should enforce the incrimination of such actions for the following reasons:
1. It keeps society civil and not animalistic. A people left to themselves without rules or guidelines will naturally drift into a state of savagery. Having sex with everyone without limitations is a mark of that.
2. It supports the family unit. The family unit is the foundation that the rest of society is built upon. By ensuing that incest is ethically wrong, we can encourage the family unit to function properly in a way that betters the the rest of society.
3. It supports the sanctity of sexual intimacy. Sex is sacred act between two people who love each other more than life itself. It is a time for these two people to give their entire self to another in a display of sacrificial love. To say that sex is something that is solely meant to give us pleasure (although that is a very nice additive) is to incredibly degrade it. As a society, we should seek to uphold the sanctity of sex.
I see this issue as another product of moral decline within our society. As a student of history, I notice that anything to do with sexual immorality is a mark of a morally deprived nation. It would greatly sadden me to see my nation sink farther than we already are.