When Group f/64 advocated "straight" photography, they didn't mean "unmanipulated." What they meant was that they rejected photographs that were intended to be deliberate emulations or derivations of other art forms. From the Group's manifesto:
"The Group will show no work at any time that does not conform to its standards of pure photography. Pure photography is defined as possessing no qualities of technique, composition or idea, derivative of any other art form. The production of the "Pictorialist," on the other hand, indicates a devotion to principles of art which are directly related to painting and the graphic arts."
The whole text of their manifesto is easily found using google (I don't want to link it here without permission).
Declarations like this have to be understood within their historical context: this was written at a time when photography was fighting for "legitimacy" within the world of fine art. Group f/64 was rejecting the idea that nothing made with a camera could be the "equal" of things made "by hand." They were also rejecting the idea that in order to be "art," photographs had to LOOK like the prevalent "art" of the era. In some ways, these ideas still apply, but in others they are quite dated: I doubt if anyone would seriously argue today that Adams, Weston, Cunningham, etc. were not artists or that photography cannot be art. Weston wrote a lot about the subject of "truth" in photography in his Daybooks, which are still great reading, IMO. We each have to decide for ourselves what to make of these ideas and how to integrate them into our own work.