Putting past my personal view that it's a degenerate act, I believe it's frowned upon for the sole reason it puts the possible child at higher risk of complications in early and later life. It could be a potential danger to the human species if accepted or practiced on a larger scale over multiple generations. It's rational for people to think, even in the older times that it's not desirable for the future of the population. There are also degrees of incest, like siblings-parents-offspring or cousins-cousins, so I don't know which you are discussing but I will try and cover the whole of it.
Also let it be said that I am not dealing with hypocrisy because it's not relevant to the issue about incest. Now my personal view:
I would not care if countries legalized incest because it would not affect the human species on a large scale because it would maintain the global view of it being a taboo. Incest has been likely made illegal in your country as one of the earliest laws in its creation to maintain this view ages ago. However I don't believe it really needs to be this way when it can destroy private relationships that people don't need to be involved in. On a microscopic scale about one family, say, it would help their relationship grow no matter how degenerate it may be in other's eyes. There isn't a justification for it being illegal when it's already ingrained into western, or really, global culture in the modern era. Though, there would need to be a different way to approach it such as regulations it were to be legalized, due to the damage it can cause if there are a surprisingly large amount of degenerates in the human species.
On a tangent, I don't believe that the people who get off at incest are on that same level. It's a psychological fetish and can occur in people who live without siblings and would likely never do it in real life. I personally don't follow that specific fetish but I would say that those people aren't on the same level as the people who commit it physically. I am not calling people who are interested in incest though media are degenerates, not nearly on the same level as people interested in minors.
The big thing to take away is that on the other hand, there is absolutely no justification for allowing incest to be not frowned upon. The same way free speech is maintained, people can say whatever they want and not face government's intervention, but will instead face opposition of other people's views. It is generally accepted, healthily accepted by the human population that incest is not a desirable trait no matter how you spin it. Besides the current legal dispute about how it can damage relationships, it's not in any way a positive thing overall.
You can't say that there isn't anything wrong with incest when it is proven dangerous over the course of generations, and as a population across different languages, nationalities or governments we are able to maintain the healthy view that it is taboo whether you like it or not.
