Fortunately, not all incest supporters are like that. I, for one, am pretty neutral on incest. But as long as the relationship is between two consenting adults, I don't really care if it's incestuous or not.
In my honest opinion, if the two didn't know each other, were in a situation where they were separated and couldn't develop platonic feelings for one another, then I'd totally understand; but, if they knew each other from the start, that's where I have a problem with it.