I'm sure others will cover the female representation angle better than me, but the other issue I have with the 'I'll take the form you're most comfortable with' idea is that it removes the primal fear of dealing with a huge monster than is sentient, highly intelligent and could easily either step on you or eat you just on a whim. Imagine if Smaug was a slim bloke with red hair and a deep voice, or the Balrog a tough looking chap with a flaming sword. Suddenly the whole power dynamic shifts as, while you still would have zero chance against them in hand-to-hand combat, that fear has been removed, they seem more beatable, their movement limited, their abilities and sheer bulk limited, their intelligence less alien. You're less like prey, less insignificant in proportion. To humanise such monsters removes primal fear and makes the player feel more powerful and less like food in contrast, which isn't always a good thing.
