Yes. In fact, there's a great British mini-series about this very thing, with a gay man falling in love with a woman (and still very much identifying as gay and not bisexual).
It was actually based on a true story that happened with the writer's friend:
"Bob and Rose involved homosexuality as a key theme. The storyline involves a gay man named Robert Gossage ('Bob') falling in love with a woman, Rose Cooper, and was loosely based on events that had happened in real life to a friend of Russell Davies. This set-up caused an uproar among some gay rights activists who felt that the premise would make it appear as if being gay was a choice or a phase, which then generated a strong counter-reaction by bisexuals who called the criticism unfair."
http://www.youtube.com/watch?v=9iOUIDqhukA
You should really watch it if you can- very good stuff. The simple fact of the matter is that, while most people find love within their sexual orientations, sometimes love is gender-blind, and you really do fall in love with one person in spite of their gender, whether it's a straight person falling in love with the same sex, or a gay person falling in love with someone of the opposite sex (oddly enough, nobody has a problem with the former, but everybody screams and moans about the latter).
Another real life story:
* Director Greg Araki: "Araki self-identified as gay until 1997, when he entered a relationship with actress Kathleen Robertson, whom he had directed in "Nowhere". The relationship ended in 1999. Since then, he has been involved mostly in same-sex relationships."
