^Well, I can't talk for everyone, but I ship them because they have good chemistry, they look great together, they love each other unconditionally, and more important of all: they're not real.
I, myself, don't really like incest, especially on real life, but this ship is a guilt pleasure of mine.
I wouldn't take it seriously if I were you. People won't stop shipping it just because someone doesn't like it. But if you do want to take this seriously, remember that in history there are a lot of incest. In some cultures it was pretty common to marry royal people of the same family so "the blood would stay pure" or whatever. Egypt was a big example of that.
I recomend not looking at pictures/manga/stories/etc. that have this tematic. At least that's what I do when I don't like the idea behind the work. It's effective and I don't get stressed or grossed out.