From the author of What's Wrong with Homosexuality?
So some people say, 'Well, I just don't understand bisexuality.' Let me take a moment to explain.
Some people are attracted to both men and women. That's it.
No, really, that's it. Doesn't mean they're attracted to everyone because that would be exhausting. Doesn't mean they're confused; doesn't mean they haven't decided yet.
.... some of us in coming out as gay people weren't ready to say we were gay, so we said we were bisexual as a way of kind of dipping our toe in the water .... It should go without saying that there are genuine bisexuals in the world.
... If I'm in a relationship with a man, you identify me as gay. For a bisexual person, unless they're in a polyamorous relationship ... people make all kinds of assumptions about them. They see a bisexual woman who's in a relationship with a woman, and later a relationship with a man, they say, 'Oh, she was just experimenting before.' Or they'll see a bisexual man who was in a relationship with a woman, and then a relationship with a man, and say, 'Oh, he was really gay all along.'
Some people actually prefer to identify as omnisexual or pansexual rather than as bisexual, because it gets them past the whole binary construction of gender....
(Lots of other good stuff, too, in 7 minutes!)
I also recommend Prof. Corvino's lecture "What's Morally Wrong with Homosexuality?", an excellent demolishing of common arguments both religious and "secular", where he also recognizes that "bisexual people are not half kicked out of the house for being bisexual, or half fired from their jobs, or half harassed, for being bisexual." It's a full hour-long lecture, but worth listening to, especially if you use something like listentoyoutube.com to extract the MP3, and listen in the car or gym or wherever.
That. Was. BRILLIANT!
He also mentioned the idea of "opposite" genders suggesting that people might be attracted to just one or the other -- but he likes the "opposites" of hot tea and iced tea, both.
In "What's Morally Wrong with Homosexuality?", he mentions sexual orientation as a narrow aspect of the "types" one's attracted to:
I have a certain sexual orientation; what is that? I'm attracted to people of a particular gender; that's true. I'm also attracted to people of a particular age range, body type, personality type, certain kinds of senses of humor. All of these things make up my sexual orientation, broadly understood. But when we talk about sexual orientation, we focus very narrowly on the gender of people that you're attracted to, and then we divide everyone into these nice neat categories: there are heterosexual people, and there are homosexual people.
Then there are bisexual people, and they mess up our neat categories. Everywhere I go people say to me, "I just don't understand bisexuality." Let me take a little time to explain it, 'cause it's not a complicated concept, really.
Some people are attracted to both men and women. That's it. [etc.]
His retort to a Catholic priest who argued that homosexuality is wrong because if everyone were gay there would be no society [no future generations]: "If everyone were a Roman Catholic priest, there would be no society either."