Actually, as far as First Cousins ... the majority of cultures see no problem with them in dating relationships. In fact, in many cultures it is normal. Here's the reasoning: When you are young, you hang out with your cousins far more than anyone else your age. So when you start getting interested in dating, you are first attracted to your cousins. > It's still unusual for them to marry, but there isn't the stigma there is in much of the US.
And the young age for relationships has a lot to do with life expectancy. It used to be quite common in the US to expect people to marry before 20. Now that we live another 30-40 years, and don't rely on large families to support us in old age - we don't need to marry as early. However, that doesn't change the fact that biologically people are ready to bear children in their teens. Of course, teenagers are no longer expected to fulfill other adult roles in society now (such as having jobs) ... so increasing the marriage age has more to do with that, than any "moral" reasoning. > Maybe, if our life expectancy increases to 150 years, those generations will look at people who marry 30-year-olds as perverts. LOL