Dangerous Ages by Rose Macaulay

Next day, at three p.m., Mrs. Hilary told Mr. Cradock all about it. Mr. Cradock was not in the least surprised. Nor had he the slightest, not the remotest doubt that Nan and Stephen Lumley were doing what Mrs. Hilary called living in sin, what he preferred to call obeying the natural ego. (After all, as any theologian would point out, the terms are synonymous in a fallen world.)

"I must have your advice," Mrs. Hilary said. "You must tell me what line to take with her."