Sperm banks pay for the sperm and some men donate steadily for the money...their 'progeny’ have a 50-50 chance of meeting and marrying at some point, never knowing they are related i.e., brother and sister; father-daughter; mother-daugter etc. The protection against this happening isn't there.
Where do you get that probability? It is extraordinarily high.
Since the beginning of time, both men and women have had affairs, often resulting in pregnancy. Children of these affairs have the same, if not higher, chances of meeting, since their parents may well be in the same social circles. There are many, many more of these kids than kids from sperm banks. So far we have not seen devastating biological consequences of this known human behavior.
Sperm banks fill a useful function for some married couples who suffer from infertility. The problems arise when they are used to allow lesbian couples to have children "of their own".