Incest Question

I just browsed through a recent quibbler_report entry and had a moment of intense confusion at the following description of a fic:

(Andromeda/Regulus, R, incest)

Now this isn't an attempt to diss the q_r or a particular fic or author (the fic itself has an incest warning as well), but something I've seen quite a few times recently.

*But*... they're *cousins*. They could, technically, marry, have children, everything. Well, at least hereabouts, and I haven't really heard of a country in Western Europe that would label a relationship between cousins incest. Not even in the US, weirdly place that it is when it comes to sexual morals... Or am I mistaken about this? Is there a greater taboo about cousin-ly marriages than I've been aware of?

*scratches head*

ETA: I spoke too soon - US sexual morals *are* weirder than I'd have expected :): So if a relationship between first cousins actually qualifies for incest for some American states, the warning makes sense for at least part of fandom... Anyplace else? *is curious*