I think laws forbidding cousin marriage are unfair and should be the next target of civil rights activism.

I've seen it argued somewhere that the acceptability of cousin marriage in society also relates significantly to the presence of corruption/nepotism/tribal thinking in that society.

Incest laws in general makes no sense except as a eugenics measure.

Besides eugenics issues, I'd guess that because of pre-existing power dynamics incest probably has much more of a potential to be psychologically unhealthy than most non-incestuous sexual relationships -- much like teacher-student / doctor-patient relationships are also frowned upon, except that the familial dependency is even stronger.

It puts all the eggs in one basket after all (psychological/financial support from biological family, psychological/financial support from significant other) -- if these are the same people, who do you turn to when you have problems?