Did the US forces use tens of thousands of Japanese women? Yes. The New York Times article Fearing G.I. Occupiers, Japan Urgesd Women Into Brothels indicates the number of Japanese women involved were in the tens of thousands:
Internal documents of the front organization, the Recreation and
Amusement Association, show that 55,000 women served in it. The figure
includes some office workers, but the great majority worked as
Were those women slaves? There are allegations of some women being forced:
In a few cases, the authorities may have forced Japanese women to work
in the brothels. One Japanese book asserts that female factory workers
in the city of Kawasaki were trucked to a brothel for Americans and
given a speech by a man who said he was from the Interior Ministry.
"You should be proud to be given this mission," he reportedly told the
women, who had been told only that they would work in the tourism
business. When one woman tried to escape, she is said to have been
beaten and her right eye gouged out.
In another case, a women's corps affiliated with the army in Saitama
Prefecture is said to have received an order on Sept. 9, 1945,
dispatching the members to four brothels in Tokyo. The order, from the
Interior Ministry, reportedly said: "You should bear the unbearable
and be a shield for all Japanese women."
The sentence quoted is the only sentence in the article that mentions sex involving US forces and Japanese women. This makes it difficult to know exactly what the article is alleging or not alleging.
As the Alternet article doesn't explicitly allege that individual clients knew that the women were sex slaves, or that the US government knew the women were sex slaves, I won't investigate whether there was US complicity in the enslavement.