Sperm Donation… Inbred Nation?

I was just thinking about the ethics of sperm donating.

Here is a story to explain my worries.

MR A donates his sperm (which goes to MRS B).  At the same time or sometime after MR A has sexual relations with MRS A and produces MASTER A (in this  time MRS B produces MISS B). One day MASTER A and MISS B meet each other, have sexual relations and produces…. oh good lord inbreeding has occurred AAAAAGH!!!

Just a thought and highly unlikely… but still possible!