I think it’s mostly a combination of people being against incest on principle, and people just not liking Kirino. Both of which I think are silly, but what can you do.
Source: neogaf.com

I quite enjoyed that part of the ending, actually.

I still don’t see a problem with mutually agreed-upon brother-sister incest. In fact, I’d have been dissapointed if they’d bailed out and taken the safe route.

Source: neogaf.com