Look at it this way. There is a measure of understanding in all relationships. You act differently around friends than you do with your significant other and different still with parents and even differently with relatives.
Now we all know of cases where relationships have changed. Lovers grow to hate each other and friends grow to become very affectionate etc etc. The thing is, when you change the relationship, the rapport between the two of you change. If two friends start going out, they become a couple. People don't consider them friends anymore, but something different(albeit, very similar). In fact, if you ask about a couple like that, more often than not, you'll hear them described as "they used to be friends". If that couple were to become very hostile tho and break up, they are no longer considered a couple and have become something different. The dynamic of their relationship has changed.
With incest, however, you have a condition that exists in paradox to itself. You have a specific familial relationship with your mother, your father, your uncle, your cousin, yadda yadda yadda. The thing with these kinds of relationships is, they are immutable. Short of altering the laws of physics and warping space-time, your father will always be your father, your mother will always be your mother. The reason this is the case is that you had no control over the creation of this particular relationship, ergo, you cannot change it. So let's say that I started to court and fornicate with my first cousin or something. We are still first cousins even afterwards and I have breached the relationship that exists between us. You could consider us lovers or a couple, but that particular relationship must exist with the fact that we are still family. The two are wholly incompatible with the importance of family taking precedence. So in the end, the situation is not as normal where we have changed the terms and condition of our relationship, but rather, both of us are in breach of that relationship. And if people everywhere didn't respect the boundaries of other people, this would be very bad. Why? Because that respect is a major part of what keeps everyone in harmony in today's society. If U took another social order, however, like the nobility of france in the 16th century, where familial terms took a back seat to courtship, you have another story altogether.
my 2.5 cents and a euro.