OK, first things first. Megan has no external male sex organs:
If you study that picture you'll realize that Megan showed off more than she intended. If she was male the base of her penis would have been clearly visible.
The conspiracy theorist would respond that Megan has therefore undergone a drastic program of cosmetic surgery and hormone treatment to give her female body characteristics.
But that wouldn't explain this:
It is not possible for a man to have a hip-to-waist ratio like that. Even if he were to have his lower ribs removed, and receive regular silicone injections, it would be impossible to create such wide hips in relation to a narrow waist.
Nor could this be achieved by hormone treatment: during puberty estrogen causes a girl's pelvis to physically widen - the bones change shape. This either happens during puberty, or it does not happen at all. All that male-to-female transsexuals who take estrogen supplements in adulthood can hope for is a slight redistribution of fat, but in any case this is more likely to accumulate at the waist, thus skewing the classic hip-to-waist ratio.
The only insane conjecture one could possibly hang on to, is that, before Megan was 9 or 10 (by which time significant amounts of male androgens would have been released into her body), her natural male sex organs were removed and replaced with an ongoing series of estrogen supplements. But even if one was deranged enough to believe this, it would simultaneously mean that any female celebrity who has never been known to have a child could reasonably be suspected of having been a boy in childhood. More importantly, such an individual would be more "woman" than "man" anyway, having no remaining male sex characteristics.