I'm not sure that's true. It is a frequent report by "straight" CDs that they do experience sexual attraction to men when fully in the female role by dressing and interacting. I know it's tempting to analyze the phenomenon as acting out a fantasy or tweaking one's autogynephilia (a problematic and controversial hypothesis at best), but the phenomenon is very real for some. I view it as the social training that tries to pound any femininity out of us as boys producing such an inhibited and suppressed sense of self that, without the "permission" we give ourselves by being fully in the role, something inside us just won't allow us, presenting as "men", to find males sexually attractive. Thus, we're quite different from gay men who love being men and love being with men. And quite different from CDs who love their straight sex lives with wives or SOs but just like to dress up.
Case in point: Samantha's candid and clarifying comment this morning, which I hadn't gotten to before posting the foregoing comment. 'Nuff said.