Second cousins are also the children of your first cousins :D
Funny stuff, actually. We've been talking about this in biliogy recently (we're doing a unit on genetic right now) so this question has come up for me lately. This is mostly because we have to a response on an article entitled
Go Ahead, Kiss Your Cousin and it does raise some rather interesting point.
Such as, up to 80% of marriages in all of history have been between first cousins or closer. Or at least, that's their approximation, mostly due to the fact that for many years you would marry someone within five miles or so of where you lived (however for you could walk and back in your day off).
Oh, and just to put something in perspective. Even with your
first cousins, you share
under 50% of the same DNA as them. It really doens't raise the chances of birth defect or any such thing very significantly, if you want to look at it from that point of view.
But since we're talking mostly ethically here and such, I suppose I'll have to talk about that. I mean, I really don't know if I'd want to marry any of my cousins, first or second, but I think that's more personal than anything. I know all of my cousins, and I just wouldn't really want to marry them. (Not to mention the closest one to my age is about 5 or 6 years my junior, and I haven't actually seen him since we were pretty little :P)
So yeah, though I wouldn't do it personally, I don't really see that much of a problem with it among second cousins and such. First cousins might be a bit odd, but it really depends on how you were raised. Though, thinking about it, half siblings don't really share much more DNA with you than your first cousins (it's still under 50%).
Anyways, I ramble :D