Scientifically speaking, what's wrong with incest?

As long as you don't produce a defective child, it's fine, right? What if you don't produce children at all? If your sister is on the pill or if you always use a condom, there's nothing wrong with that, right? What if you have an incestuous relationship with your father? No child is produced, so there's nothing wrong with that, right?

What are your thoughts on incest? How likely is it to produce a defective child, and is there anything wrong with it if you don't have children?