Bisexual?

I've just finished watching the film for the first time and that question crossed my mind as well. He did seem to genuinely love Constance. I was especially surprised in the scene where she visited him in prison and he said he had always loved her. The fact that she didn't want a divorce after all they'd been through seemed to prove that perhaps there really was more to their relationship than the façade it appeared to be throughout the film. I have to say though, Constance must have been the most naïve woman alive not to realize that Oscar and Bosey were lovers. Then again, maybe she knew all along but didn't make a fuss, so as to spare their children and maintain the family's reputation. I do think there is a possibility that Oscar could have been bisexual but from the way this film portrays him and from all I have heard about his life, it seems more likely that he was a gay man having to live a double life because of the era in which he lived. A truly fascinating figure who left behind an enduring legacy of brilliant writing.

You have to remember that, culturally, there was at this time only beginning to be a concept of heterosexuality vs. homosexuality, of sexual orientation. Previous to this, sex was something that you did, not something that society considered you. For instance it was not considered abnormal for men and women to have 'passionate friendships' with members of the same sex, and younger boys were frequently taken as sort of boyfriends by older boys at the upper class schools.
And at the time, especially among the upper classes, no one married for love, or very few at any rate.
It could very well be that he was what we would consider bi. Sexuality isn't a 1/0 switch, it's a continuum and even if he was primarily attracted to young men it's quite possible that he was really in love with his wife. More likely, he had great affection for her, and her children, and the life that they represented that he really couldn't be part of but probably admired.