Had to address this separately. This is absolutely false, on two different levels. On one hand, as far as the laws go, there are many non consanguineous relationships which are prevented from marrying, the relationship being legal and not by blood (laws vary by state). There is not a single biological issue that comes from this kind of marriage, at least no more than any other couple not blood related.
On the other hand, as noted earlier, marriage does not cause sex. Sex occurs quite frequently outside of the legal state of marriage. If an opposite sexed consanguineous couple were going to have sex, do you really think they would bother to wait for a legal marriage certificate? As I have said before, the only reason to seek a legal marriage is to obtain the benefits of the legal institution. It is not to obtain permission for sex.
Furthermore, your argument also fails due to the fact that a same sex consanguineous couple or an opposite sexed consanguineous couple where one or both are sterile, could be married AND having sex and would never produce a single biological problem, as you put it.
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