I thoughtI had made it clear that I don't subscribe to your idea that positive discrimination is the reason for allowing fostering by same sex couples, Dave.
I repeat, the shortage existed before single sex fostering was legal, and I see no evidence to suggest that the removal of single sex households would suddenly bring forth more hetero foster parents. It would IMHO put many children back into institutional care, which has its own history of systematic abuse, as a visit to any newspaper archive will easily confirm.
Children have suffered abuse at times in every conceivable environment, and it is wrong to seize on one case and extrapolate from that the kind of general conclusion you are asking us to accept.