Cast your mind back 25 years or so, to the time when the only people allowed to adopt or foster were hetero couples in stable relationships.
Indeed, the time when there were no officially recognised same sex couples, as it was illegal.
There were massive shortages of foster parents, with thousands of children in institutional care all over the country.
Do you really believe that the relaxation of strictures to allow fostering by previously banned individuals brought, in its wake, a sudden huge increase in the number of hetero couples wanting to foster.
I think your (perhaps understandable) misgivings about same sex couples fostering is blinding you to the truth that there have never been enough foster parents available, and what you see as positive discrimination is, in fact, nothing of the sort.