Cousin marriage is not prohibited in the Bible.
I didn’t say it was, but it has been known for centuries that inbreeding was bad, and during modern times, is recognized as universally bad.
I do see the point you make, and I understand.
I think a better way to put it is, the issue isn’t that first cousin marriage in and of itself is very very bad (though it can be if someone has certain genetic makeup already that will make something recessive express itself) the bad thing is that REPEATED and PERVASIVE first cousin marriage IS indeed bad, and it WILL bring undesirable traits to be expressed.
Having a first cousin here and there marrying isn’t much of a problem, but family-wide, community-wide, generation after generation is bad.
For an analogy, imagine just throwing all the genetics in a big pot and simmering off all the good stuff, leaving a concentrated morass of bad genetics more likely to express themselves.
But neither is it promoted by Bible-believing culture.