The following excerpts are from Arthur Carl Piepkorn’s article “St. Paul on Social Relationships,” Concordia Theological Monthly, 11:10 (October 1940), pp. 721-52, the first article that he published in the journal. As far as I know, he never changed his opinion on these matters. 
–Philip J. Secker 

Piepkorn cites F. J. Foakes-Jackson approvingly: “Only where Christian principles were absolutely at variance with the existing order—as regards, for example, idolatry, impurity, and infanticide—did the Church take a firm stand.” p. 725. The Life of St. Paul: The Man and the Apostle, London, 1933, p. 335.

“Because of the diversity of gifts the individual guidance of no one is to be rejected out of hand—the command is express: ‘Quench not the Spirit; despise not prophesyings’—yet all such directions are to be tested by the absolute standard of the revealed will of God: ‘Prove all things, hold fast that which is good.’ (1 Thess. 5:19-21).” p. 731.

“The Christian way as conceived by St. Paul tended to emphasize the duties of Christians and to discourage their too intransigent insistence upon rights.” p. 731.

St. Paul’s “attitude toward every variety of sex sin is uncompromisingly hostile. Tribadism (Rom. 1:26), sodomy, pederasty (1 Cor. 6:9ff.) and other kinds of homosexuality and perversion are marks of a degenerate philosophy that has completely turned its back upon God.” p. 733. On tribadism [lesbianism] Piepkorn refers in a note to Morton Scott Enslin, The Ethics of Paul, New York and London, 1930, p. 146.