Her "boyfriend" cheated on her and stole all of her money. Her boss, despite all of her hard work, has promoted a male over her. She has dysfunctional relationships with the males in her family, possibly emotionally abusive. It doesn't have to be full emotional beat-downs, but just the little things. Add all of those together, and you get a woman who distrusts men.

"You need me. You wouldn't be much of a hero without a villain. And you do love being the hero, don't you. The cheering children, the swooning women, you love it so much, it's made you my most reliable accomplice." -- Lex Luthor to Superman, Question Authority, Justice League Unlimited