Relationships - how close is too close?

In the past, we've all probably seen cases where first and second cousins
marry. [First cousins are permitted to marry only in some places.] I even
once saw an uncle-niece marriage (but am still wondering why that was
allowable let alone allowed). However, my question does relate to MORE
DISTANT relationships and MODERN society.

I recently met someone whom I've been dating, and we compared our family
histories as we both have an interest. We're not related within 10
generations but it appears (currently unverified) that we might have two
common families about 12-15 generations back.

I'm not worried genetically. Since people only have about 30,000 genes, after
15 generations, some ancestors are guaranteed to be non-contributors (32,768
people without regard to duplicates due to other cousin marriages), while at
most only two ancestors are guarenteed to be contributors [the matrilineal
line for the mitrochondial DNA, and for males, the patrilineal line for the
"Y"]. Therefore, with our distance, our commonality may be not significantly
more than any two people not related within 20 generations but still of the
same race. I believe that statistics would guarentee that we share 8 genes
minimum out of 30,000 without further specifics to genetics (e.g. only 8 blood
types, limited hair colors, skin color, etc.).

My question is not genealogical, but more of the social or moral view. At
what point (degree of cousinhood) would two distant genealogical cousins be
considered the same as if not related? First and second degree cousins are
usually called such, and sometimes third are, but socially, is that where the
relationship plateau cuts off? Except for genealogical topics never have I
heard anyone being compared as fourth cousins or more distant....