I don’t get the logic behind this.
No actor could publicly say that he frequented brothels. None of the gossip magazines would print reports of actors going to brothels. Everybody knew they existed, but they weren’t anything you could talk about in public.
So how could this work? The movie audience could never know that Stewart went to Mayer’s brothel. Or to the multiple other brothels in Hollywood at the time. I could maybe see Mayer financing a brothel so that his stars had a safe place to go, but one that got the word out? Never. Even if the idea was to prove to Mayer personally that the actor wasn’t gay, the scheme is nutty. Mayer slept with starlets - not prostitutes - all the time. If he asked one to go with Stewart for an evening he’d have a dozen to pick from, eliminating all the expense of running a brothel. And if the actor did give the girl a good lay, so what? Many were bisexual. Only the most extreme homosexual actors couldn’t give a good performance if they had to.
This article on Hollywood sex cites William J. Mann’s book Behind the Screen: How Gays and Lesbians Shaped Hollywood, a solid source, as saying that Ramon Navarro and Billy Haines “were scolded by MGM’s producer, Louis B. Mayer, for patronizing a male brothel on Wilshire Boulevard.” That’s miles away from the actions of a maniac who made his actors fuck on cue to prove they were hetero.
The whole thing is idiotic. I’ve read dozens of books on classic Hollywood, including many recent ones that have examined the large gay community, and never come across a sentence that backs this up. Marc Eliot’s book on Jimmy Stewart gets slagged on Amazon, too. I wouldn’t trust him as a source, especially since in the decade since the article was written, apparently no one else has backed him up on his assertions.