Ok, how about homosexual incest? ;D
(purely a hypothetical added facet to the discussion, from an only child.)
First cousins have a 2-3% chance of conceiving a child with a birth defect, but I think that's only the case if there hasn't been ancestral incest for several generations, but I'm not 100% on this. Incest is just a taboo unless offspring are conceived and every successive generation kept it in the family then it becomes redneck-royalty.
I live in Kentucky, which has a reputation for marrying cousins. I think it is a pretty outdated stereotype, as I don't think it occurs very much today, but back in the 1800s and very early 1900s, it was quite common. In fact, I have a 3rd great grandfather in my family tree that is my 3rd great grandfather three times over, due to cousins marrying cousins.
Back in the 1800s, if you lived in a rural area, there simply weren't a whole lot options available for sexual partners. Two or three families often moved to an area and after a couple of generations, almost everyone in the community was fairly closely related. People didn't travel much then and as long as your family had sufficient acreage of arable land, you wouldn't have much reason to leave.
Like others have said, as long as it is consensual and no children are involved, I don't really see an issue with it. There's an "ick" factor that, like that of homosexuality, is impressed upon us by some in society, especially by religion (even though incest is prevalent throughout the Old Testament).