It's well known these days, that females become more masculine and men become more feminine. If you believe that most of these confused 'men' aren't bi- or homosexual, than you must be living in lala land.
In fact..., I knew a guy who was married for many years, but later he became estranged from his wife. Needles to say how he found his true identity...
People have a poor knowledge of homosexuality and bisexuality and, stereotypical portrayals have contributed to this misunderstanding and conflation of sexual orienation, gender expression and gender identity.
Psychology has studied the phenomenon and, to make it short, 8 out 10 men who crossdress are heterosexual men. It is generally called "transvetic fetishism".
Regarding masculinity and femininity, what's masculine? what's feminine? It is hard to give an answer because these concepts vary from culture to culture; there isn't a universal definition for them. Also, all men have feminine qualities and all women have masculine qualities. No man is 100% masculine and no woman is 100% feminine.
There was a BBC documentary on heterosexual men who crossdress on youtube. But I haven't been abale to find it. There is this one from a Spanish-speaking man who talks about transvestism being heterosexual.