There was certainly something going on, that's for sure!
In the meantime, I'm going through google scholar with the search term "Victorian sex guide men". Any other ideas for me to try after that?
Some random books.
Sex knowledge for men, including a program for sex education of the boy, by William J. Robinson (1929 - could be a very late reprint if a "classic")
Problems and principles of sex education; a study of 948 college men [by] M. J. Exner (c1915)
The debilities of our boys and the early decay of our young men - by James C. Jackson (1872)
Developing into manhood, designed for use, under adult leadership, with youths from fifteen to eighteen years of age [by] Winfield S. Hall. With "Personal thoughts" by George J. Fisher. 1911
Education in sexual physiology and hygiene; a physician's message, by Philip Zenner. 1910
Education with reference to sex - by Charles Richmond Henderson. 1909
Education with reference to sex agencies and methods by Charles Richmond H . 1909 (agencies and methods?)
Education with reference to sex pathological, economic, and social aspects -Henderson, Charles Richmond. 1909
Innocence and ignorance / M.S. Gillet ; translated, with forward by J. Elliot Ross.1917 Catholic
Marriage and the sex-problem, by Dr. F. W. Foerster ... Translated by Meyrick Booth. 1911
Pictorial review's correspondence course in sex-education [electronic resource] : a series of six talks on anatomy, physiology, hygiene, and habits / by Louis Henry Levy. 1916
Private sex advice to women. For young wives and those who expect to be married. This book was written so as to give enlightenment to those entering into wedlock so their married life will be one of happiness and pleasure. Armitage, Robert B. 1918
Prostitution and sex education. Hatch, Willard Packard. 1910
Sanity in sex, / by William J. Fielding. 1920
Sex discipline for boys in the home - by William Byron Forbush. 1914
Sex-education; a series of lectures concerning knowledge of sex in its relation to human life, by Maurice A. Bigelow 1921
Sex secrets - Armitage, Robert B. 1918
Sex training in the home; plain talks on sex life covering all periods and relationships from childhood to old age, by Winfield Scott Hall.1918
Sexual hygiene; compiled from books, articles and documents, many not hitherto published. By the editorial staff of the Alkaloidal Clinic.1902 (score! look at the date! this is also the second edition.)
The social emergency; studies in sex hygiene and morals, ed. by William Trufant Foster ...with an introduction by Charles W. Eliot. 1914
Social travesties and what they cost, by D. T. Atkinson. 1916 (about STDs)
Teaching truth. By Mary Wood-Allen. 1923
Towards racial health; a handbook for parents, teachers, & social workers on the training of boys and girls, by Norah H. March ... with a foreword by J. Arthur Thomson ... and illustrations by Jessie M. Lawson. 1919
Training of the young in laws of sex / by Rev. the Hon. E. Lyttelton.1912
What a father should tell his son - by Isabelle Thompson Smart. 1911
What a young boy ought to know / by Sylvanus Stall. 1905
When a boy becomes a man, a little book for boys, by H. Bisseker. Revised by the Council of the Medical Oficers of Schools Association of London, England. With a foreward by Edward Bok. 1911
A word to the wise ... Issued by Edward Hyatt, superintendent of public instruction. 1913
The Poison of Prudery - Walter Matthew Gallichan
The Great Unmarried - Walter Matthew Gallichan (about Celibacy!)
Victorian secrets : an anthology of Victorian erotica - edited by Davina Charleston
Ethnological and cultural studies of the sex life in England, illustrated, as revealed in its erotic and obscene literature and art; with nine private cabinets of illustrations by the greatest English masters of erotic art. Translated and edited by Richard Deniston. (I'm not clear on the original date of this, but the catalog entry makes it look like a subscription work like Burton's Arabian Nights - probably because it was so "pornographic"). Contents look very compatible with MM, and would make for hilarious fic, IMHO).
Ethical hygiene: solid facts boiled down : six live books on the laws of health : physical hygiene, mental hygiene, moral hygiene, sex hygiene, maternal hygiene, and social ethics / by E.L. Rector. (1917)
Pleasures taken : performances of sexuality and loss in Victorian photographs - Carol Mavor
---I would just randomly Google titles you like to see if you can get more info on them, because many of them may be of Victorian provenance, but the library only owns the later edition printed by an American press, etc.
SECONDARY SOURCES
Modernism, Mass Culture, and the Aesthetics of Obsecenity - Allison Pease
Eros and modernization : Sylvester Graham, health reform, and the origins of Victorian sexuality in America -Jayme A. Sokolow
The dark angel : aspects of Victorian sexuality - Fraser Harrison
The worm in the bud; the world of Victorian sexuality - Pearsall, Ronald
The physician and sexuality in Victorian America - John S. Haller, Jr., and Robin M. Haller
Bloomsbury, Modernism, and the Reinvention of Intimacy - Jesse Wolfe.
Altered states : sex, nation, drugs, and self-transformation in Victorian spiritualism - Marlene Tromp
Slumming : sexual and social politics in Victorian London - Seth Koven
Mapping male sexuality : nineteenth-century England - edited by Jay Losey and William D. Brewer
When passion reigned : sex and the Victorians / by Patricia Anderson
The destroying angel : sex, fitness & food in the legacy of degeneracy theory, Graham crackers, Kellogg's corn flakes & American health history - John Money
Dangerous sexualities : medico-moral politics in England since 1830 - Frank Mort
Governing pleasures : pornography and social change in England, 1815-1914 / Lisa Z. Sigel.
Banishing the beast : sexuality and the early feminists - Lucy Bland
The spinster and her enemies : feminism and sexuality, 1880-1930 - Sheila Jeffreys
I thought Victorian Sex Education might be a better key phrase, but not getting much further. A little better luck with "sexuality". The library catalog entry is "sex customs". Sex Instruction worked if I didn't use "Victorian" with it. Many of the primary sources came from the "HathiTrust Digital Library".