So?
Plus, they did not have on average much more than 2 that made it to reproductive age, otherwise the population would have risen much faster.

Anyway, populations in the past had phenomenally high consanguinity. Brittany for several centuries was isolated enough that marriages had of necessity a degree of consanguinity that was similar to first cousins -and indeed marriage between cousins was pretty much the norm. Nevertheless, the rise in genetic disorder, while statistically significant, was too small to be practically detectable (a tiny effect can be significant when the sample population is huge).

It's actually an interesting case because the increase should have been higher based on mere genetic proximity. The most likely explanation was that there were more early miscarriages (ie, before the woman knows she's pregnant), and that they would be more likely when the embryo has a genetic disorder. I don't have a link to the study but I have it in a book -in French, though- Histoire de la notion de gène.

Even today, there are huge pockets of genetic proximities. Laos, which I know well enough, is a clear example. Most of my in-laws married cousins (not necessarily first cousins), some even double cousins (yes, genetic proximity equal to siblings in that case). Yet, despite the pretty big sample population (BIG families), the only disorder I've clearly noticed is a stronger tendency to vote UMP than one would like.

Incest is in the mind -and by that I don't mean to diminish it at all. I don't like, for example, the recent tendency of parents to kiss their children on the lips -and indeed, I've seen adult siblings doing it here in London. I wouldn't mind in Russia, but here it means something else and I don't reckon it's a good idea.

Earth provides enough to satisfy every man's need, but not every man's greed. Gandhi

Anyway, the number of children per couple has nothing to do with it. Each individual child necessarily has one biological mother and one biological father. Going back over the generations, it is numerically impossible to suppose that the parental couple in every case was exogamous. Only endogamy (and polygamy in cultures where it was practised) can reduce the number of ancestors (for example, first cousins have the same grandparents, two instead of four). We are all the descendents of, at least, cousin marriage.

As for Thomas's point about isolated small tribes, it's true that there are often rules to enforce or encourage exogamy, or channel endogamy by a clan system, for example. But

Thomas:

Those tribes that did not do at least one of them failed to reproduce.

is a supposition. It would be interesting to see some hard evidence for it.