The book Gay New York claims that many men would solicit gay men because because many female prostitutes would decline to do oral. Is this true and what factors might have contributed to prostitutes being able to be so selective?
According to Sin in the Second City: Madams, Ministers, Playboys, and the Battle for America's Soul by Karen Abbott, oral sex was widely available and popular. Contemporary Chicago Vice Commission reports also made it sound relatively common.
Other accounts that I have come across suggest that oral sex was specifically encouraged by business owners, who wanted to minimize sex worker pregnancies. Remember that this was all well before the availability of modern contraceptives.
Modern day sex workers also state that oral is the most popular service for clients. I doubt that things have changed that much in the 'world's oldest profession'.
Did the book have any citations or a reference section?