Why is Incest Morally Wrong?

From a moral relativism point of view, the question is not "why is it wrong?" - the question is: “why do so many people feel it is wrong?” Just the thought of it makes normal people feel disgusted. Armed with that information, we can try to figure out why people feel that way. 

Why do so many people feel incest is wrong and avoid it? Researxh shows that growing up in close proximity with people - even if you are not related - inhibits sexual attraction. More than that - it's a turn off.  It also destroys important social, familial relationships and it can therefore be destructive to the family unit (see the Westermarck effect)

https://en.m.wikipedia.org/wiki/Westermarck_effect

But is that reason enough to make it illegal? The pure logic answer is it should be like adultery - morally wrong but not illegal.

The real life answer is it is illegal because it feels very wrong to most people. But... homosexuality felt wrong to most people not so long ago,  and that is now legal. Maybe incest will be legal in the future...

And so on.

As I said elsewhere, God makes life much easier.  Incest is wrong because God says so. End of story.

So, I can't make a strong case that incest is morally wrong (not as strong as say, murder).  I can say is it feels morally wrong to most people (and I can explain why). From my position there's no reason for incest to be illegal. This conclusion makes many people very uncomfortable because it feels wrong.   The morality from God argument removes that discomfort by providing a very simple reason why incest should be illegal: God says so. 

This is one reason why religious conservatives campaign against homosexuality.  Their argument is that if we allow that,  it's the thin end of the wedge. If we tolerate homosexuality it won't be long before we are all eating babies and having sex with our parents and sheep.