In Such Is My Beloved, Ronnie and Midge are flat characters, seen primarily through their impact on Father Dowling. There is no doubt that the lack of other work during the Great Depression is one of the factors that has driven them to prostitution, but others are also suggested.
Ronnie has a history of depending on men for support. She comes from a broken home and, when she left, soon entered into equivocal arrangements with various men, in which she slept with them and they gave her money for rent. This meant that when she lost her job, it seemed natural for her to turn to full-time prostitution. She also has a close personal relationship with Lou, her pimp and also her lover, which binds her to their business arrangement.
Midge is younger than Ronnie, more impressionable and easily influenced. She turned to prostitution after being abandoned by her lover. Since she could not return home without a scandal, her options were strictly limited, particularly as she has no marketable skills and is physically frail.
For Father Dowling, of course, the profession of the two women seems like divine providence, both in its biblical resonances and in the opportunity for him to save them from sin.