Every dog deserves a bone. The passage you're looking for is from Notes of a Dirty Old Man, page 145 in the new version. It's not his girlfriend ... he wakes up drunk in his room and there is a "human form" beside him. He explains how in those days he always wanted to have anal sex with a woman but they wouldn't let him. So he sees the opportunity ("it's probably Mitzi, maybe Betty. What's the difference?") and he goes ahead and slips it in.
The "woman" begins moaning and Bukowski realizes that "she" is just pretending to be asleep.Then the sheet falls back and he sees who he's boinking. Note the reaction: "... it was one Baldy M. American MALE! all went limp. I fell back in indecent horror. I fell back sick, staring at the ceiling, and not a drink in the place."
So that doesn't sound like the reaction of a gay or bisexual guy.
The incident that Carver's Dog's mentioned in the thread that mjp provided earlier, from The Day We Talked About James Thurber, is probably the only incident I remember where he allowed gay sex (the boy giving him head to orgasm.) But in the three-way later in the story, Bukowski has sex with the girl and when the boy sticks his finger in Bukowski's butt, he punches the kid in the mouth.
Interesting speculation, but I think he was probably straight. But what do I know? :confused: