I found this book at a used bookstore in Davenport, Iowa: Gods and Demons (Greek, Roman, Nordic, Celtic, Indian), published by Lyon Library sometime in the 1960s.
The cover illustrations: a very muscular Zeus, an eagle about to eat the liver of a very muscular Prometheus, and a fully-clothed Pandora about to open the box that unleashes evil upon the world.
Who was emphasizing the male beauty?
The author was Manuel Komroff, an American writer of Russian ancestry (1890-1974), author of 45 novels, editor of many more, including an edition of The Travels of Marco Polo. One of his novels, Coronet, sold a million copies.
Th illustrator was Rafael DeSoto (1904-1992). Born in Puerto Rico, DeSoto went to a Catholic seminary before moving to New York in 1923 to dedicate himself to art.
He soon found his niche illustrating the colorful, beefcake and cheesecake-heavy covers of pulp magazines, such as Black Book Detective, Weird Tales, and The Spider.
For Men Only contained buddy-bonding adventure stories interspliced with "nonfiction" about dames.
This is an interior illustration of a man being attacked by carnivorous lizards.
He also did some paperback book covers, mostly for science fiction.
In 1964 he retired from illustration to teach and work on his paintings.
Not a lot of gay connection in his life: he was married twice, and made his living in the overtly heterosexist pulp market. But still, his interest in the male form is obvious.