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Supplemental questions that I don't know the answers to: how significant are the effects of inbreeding? Are they often so bad that it would be better for an inbred child to never have existed? How does that compare to having non-inbred children with known high risks of genetic defects? To what extent can the effects be tested for (both before and during pregnancy)?
I'd be very surprised if inbreeding was so bad that careful consensual incestuous sex, with the intent of getting an abortion if pregnancy does occur, wasn't worth the risk.
Edit: Okay, inbreeding seems to be much worse than I'd anticipated.
According to this:
http://dare.uva.nl/cgi/arno/show.cgi?fid=152307
A sibling-incest child looses 28 IQ points. The risks of all genetic disorders rises massively, from p^2/4 to about p/8, so if the prevalence of carriers of a recessive disease (p) is 1%, then the disease probability would rise by a factor of 50.