I think @jmah is a really good example of why it isn’t a choice.
For the people who say sometimes it is, sometimes it isn’t, I’ll use myself as an example. Maybe this isn’t what they are talking about, but it’s the only way I can think the yes & no answer actually makes sense.
I’m not really attracted to men or women by sheer looks or whether they have a penis or not (I kind of view people as though I am looking at art, and in that sense I actually think women are prettier than men). Both a man or a woman can turn me on, so I suppose in that way I am attracted to both? I’d never describe someone as hot or sexy. Looks just doesn’t do it for me, its more of whether I like a person (which is based on personality), and then the more familiar I am with them the more easily I can be attracted to them.
For me, you might say its a choice, but its a lifestyle choice, not me actually choosing which gender I’m more attracted to. I didn’t choose to see both men and women in the same way, but I have kind of chosen to live a straight life because I married my husband. I found someone I love very much, and that person happened to be a man.
Though if we were in a different society, and gays and lesbians were widely accepted (to the point that the general public doesn’t always assume someone is straight unless otherwise indicated), I could have easily ended up with my best friend. I think the reason I ended up with a man is because of how I was raised. My parents aren’t against gays or anything. They’ve even been good friends with a couple lesbian couples, but it was never made clear to me that were a couple until I was older. Disney movies always had a guy and a girl, and well, even now, most movies are about a guy and a girl. Even though I viewed men and women the same way, it had never occurred to me that I could date either one. I had simply hadn’t thought about it. I never really dated in the past (my husband was my first) and even though I had crushes, they weren’t due to sexual attraction, but more from my own admiration of that person’s personality that led to attraction. Now that I think about it, I have felt that way about a woman. It never developed into a crush, but I think that’s just because I had assumed I was straight when I was younger.
So, you can choose your lifestyle, but I don’t think you can choose what gender you’re attracted to. Even though @jmah chose to live as a straight person for so long, she didn’t choose to be attracted to females and not be attracted to males. I hope you don’t mind me using you as an example!