I won't crack wise on this one -- As a matter of a very prudent and commonsense precaution, I would advise everyone to have a complete family medical history prior to conception. Geneticists can easily work out the odds of some horrible disease. Unfortunately, this is rarely done -- since it isn't covered by health insurance.
That said, the risk of some terrible disease befalling a kid conceived the old fashion way of bumping bellies is far greater than the risk of half-siblings meeting and falling in love.
I'm talking farther on down the line here. This has been going on for quite a while, but when women start selling their ovum, then it will get a little messy. You know it's not just single women who take advantage of sperm donors. Infertile couples utilize these "services". This occurance could certainly happen in the generations to follow.
If African Americans fear Sickle Cell in their offspring, they most certainly get tested for that genetic abnormality, and the same for Jewish couples for Tay Sachs.
As far as the old fashioned bumpin' bellies, there are a lot of jokes out there. "Daddy stop it, yer crushin' my Marlboros"! Maybe I'm being an alarmist, but people need to be prudent. What ever happened to adoption?