First one must both recognize and accept the fact that there will be some form of "discrimination" in various aspects of life, and especially interpersonal relations.
For example, many people like to walk around without shoes on. A business can "discriminate" and forbid entry because they refuse to accept possible liability for injury to someone's feet in their place of business.
People discriminate in the choice of partners, potential spouses, friends, where they prefer to go or not go, whom to talk to or not, etc..
We also allow some legal "discrimination" under the various "Equal Rights" acts, which literally require businesses, schools, etc. to discriminate on the basis of race, sex, etc.. (Quotas, who can participate...)
Now when it comes to religion, simply look at the First Amendment.
In regards to this foster care/adoption issue? IMO as long as a person has other options (not all foster care homes are run by religious organizations ya know) then if anyone involved seriously wishes to adopt...find a place that has no problems with it.