Two related thoughts come to mind.

One is that male anatomy is more familiar, and therefore presumably less intimidating, to straight men than female anatomy is to gay men.

Another is that in a heteronormative culture, men who aren't strictly monosexual are more likely to identify as straight than as gay. If what this technique actually does is make men who aren't monosexual more aware of their non-monosexuality, then I'd expect it to get more noticeable results on men who identify as straight. (I'd also expect there to be a wide range of effectiveness among straight-identified men.)