The Kinsey scale (nor any other sexuality descriptors) is not measured in a laboratory - the behavioral pattern is based on what activities are self-reported. As far as a dating/romantic/longterm partnering situation goes, honesty up front does the least overall damage.
But with transactional/hookup sex, what he brings in the moment is what you get. He can slap on any label he wants. Either you get the sort of interaction you expect or you don't. If a 'heterosexual' client hires a male provider, the self-reporting of straight is already suspect.
A little interest can masquerade as a lot, but zero interest is difficult to hide. I guess for some men, the thrill of being "the first guy who's made me come" (again, self-reported) is the gay equivalent to "ooh that's going to be too big for me!" from the Slovenian photographer's model. (Only one of those responses results in jewelry, however...)