As far as I know, marrying first cousins is illegal only in USA, for good reasons though, the genetic problems that are not limited to severe cases, they often appear as that one flaw that make someone who is otherwise perfect undesirable.
Such marriages it's not that popular in other countries either, but in some middle eastern countries, it's permitted and even encouraged, that's why Middle East is riddled with people who inherited a less than optimal gene pool.
The main reasons is old beliefs and limited education of parents, the religious beliefs also contribute to this problem.
since a relationship between boys and girls before marriage is forbidden, and there's a heavy push to marry youngsters soon after they hit the puberty, often they don't know many people outside of their family and when they're asked who do they want as their life partner, there's a high possibility that a cousins name comes up.
