Children conceived by a sperm donor are worried that they will have an intimate relationship with an unknown, blood relative.
(http://www.familyscholars.org/assets/Donor_15findings.pdf) According to Elizabeth Marquardt, Norval D. Glenn and Karen Clark, half-46%-of donor offspring... agree. "When I'm romantically attracted to someone I have worried that we cold be unknowingly related."
Realistic fear? Consider that there is some evidence that biologically similar adults are more sexually attracted to one another, especially when reared apart and this phenomena is termed
Genetic Sexual Attraction. Just ask Phil Bailey and his grandmother, Pearl Carter or Patrick and Susan Stubing, a married couple with two children. By all outward appearances they are a normal family living in Germany with one important exception: Patrick and Susan are siblings. (http://www.bizarrebehaviors.com/2009/09/genetic-sexual-attraction-an.html) So it seems that their increased worry is justified.