Allan Bérubé, in his book Coming Out Under Fire: The History of Gay Men and Women in World War II, reports that in 1921 the U.S. Army issued the first written guidelines for excluding men from service.

The guidelines excluded men whose bodies were supposedly feminine, who took part in anal or oral intercourse, or who were sexual "perverts" or "psychopaths." But, says Bérubé, these "psychiatric screening standards existed only on paper and were rarely implemented."[1]

These Army regulations distinguished "sexual perverts" and "psychpaths," and men caught in anal or oral sex, as deviators from from a supposedly nature-given or God-degreed standard of proper, reproductive, male-female, penis-in-vagina, sexual intercourse. The norm of a proper "heterosexual" pleasure sex, unlinked with reproduction, was not yet in place. The heterosexual-homosexual distinction was not mentioned or referenced.

In 1921, the Army issued expanded psychiatric screening standards that remained in effect until the eve of world war II.[2][Berube Note 14] The framers of these interwar standards drew on the theories of personality developemt to construct their list of psychiatric disorders, but the military regulations encased these new psyuchiatric concepts in the theory and language of degeneration, which ranked human beings into hierachical categories based on characteriusts that were considered inferior or "degenerate" by virtue of their deviation from a general white, middle-class, and native-born norm.

The framers of the Army's interwar standards listed feminine characteristics among the "stigmata of degeneration" that made a man unfit for military service.[3][Berube Note 15] Males with a "degenerate physique," the regulations explained, "may present the general body conformation of the opposite sex, withi sloping narrow shoulders, broad hips, excessive pectoral and pubic adipose [fat] deposits, with lack of masculine hirsute [hair] and muscular markings." A young man with a "scant and downy beard" or a "female figure" was also to be closely observed for evidence of "internal glandular disturbances." In addition to these "anatomical" stigmatic of degeneration, the interwar standards listed "sexual perversion" -- a  broad category that included oral and anal sex between men -- as one of the many "functional" stigmata of degeneration. The Army standards also listed "sexual psychopathy" as one of many "constitutional" psychopathic states -- biologically based psychiatric conditions that, through heridity, bad habits, or injury, caused a person to lose the ability to adjust to civilized society. With these 1921 standards, the Army established its first written guidelines for excluding men who displayed feminine bodily characteristics or who were sexual "perverts" or "psychopaths."  

Note
[1]  Allan Bérubé, Coming Out Under Fire: The History of Gay Men and Women in World War II (NY: Free Press/Macmillian, 1990), page 13.

[2] Bérubé, Coming Outnote, page 13, note 14 on page 292 citing Army Regulation No. 40-105, 1921. William N. Eskridge, page 392, lists this document more fully as Medical Department, War Department, Army Regulation 40-105, "Standards of Physical Examination for Entrance into the Regular Army, National Guard, and Organized Reserves," §XX, ¶93(a)–(b). See Eskridge, Gaylaw: Challenging the Apartheid of the Closet (Cambridge, MA: Harvard University Press, June 30, 2009), page 392.

[3]  Bérubé, Coming Outnote, page 13, note 15 on page 292 citing a discussion of late-nineteenth-century criminologist C. Lombroso's theory of atavastic anatomical and social stigmata, from which these Army categories were in part derived, see Stepehn Jay Gould, The Mismeasure of Man (NY: W. W. Norton, 1981), chapter 4: "Measuring Bodies: Two Case Studies on the Apishness of Undesirables," pages 112-45.