Why is incest"frowned upon"?

I am not into incest. But we always hear about how love knows no limits and walls. If a mother and son love each other on an intimate and sexual level. Why can't they love and live their life. We get into their life and we tell em they are making inbred children, Come one, is that one inbred child or couple hundred gonna ruin the fabrication of the universe itself...I mean it, why can't mother or son or any family members experience that love