Gabriela of Eleganza asked me that question the one time I met her. It's kind of heavy to mention her in connection with a light-hearted topic, but when I think of her it comes to mind.
Anyway, my theory --- based purely on my own experience and with only a modest probability of being correct --- is that the more a man masturbates, the more likely he is to fixate on lesbian sex.
Men masturbate to pictures. The sight of a woman having sex is a wonderful thing; the sight of a man having sex is not (at least to most other men). So instead of a man and woman having sex, you have a woman and a woman. The more you look at such scenes, the deeper the association grows. Pretty soon the thought of girl-on-girl automatically triggers horniness.
Or maybe not. Any other explanations are welcome.