Evidently the good Father Brown has found access to a time machine, so that he can change attitudes and morals in retrospect.
Not for the first time, he not only accepts but gives his (tacit if not overt) blessing to a homosexual relationship. This would probably have been grounds for excommunication in the 1950s. It certainly was not part of church dogma.
And was it so acceptable for Brenda to have a White boyfriend in those days? Again, the good Father just dusts his hands of any social opposition to this relationship. He has taken similar positions in other episodes as well. But when you have the comfort of a time machine, it all becomes easy.