In a way yes. I think people when discussing this topic just like to throw around words without really thinking on the issue. There's 3 parts to sexuality...
1. Which gender you're attracted to - not a choice
2. What you choose to identify yourself as - A choice
3. The gender you choose to explore sexually - Definitely a choice.
Basically you can't choose or decide what gender you're attracted to. That's not a choice. That's just a part of who you are. You can choose you're sexual partners though. I know guys who aren't really attracted to women at all, but try to live a life as a straight man for acceptance. That is a choice they made. Doesn't mean they will be happy or that they are being honest with themselves. Now choosing your sexual identity is a choice, but it's a choice that you generally make based on the gender you're attracted to. Some people will use the gender they're currently involved with though.
I think bisexuality is a confusing matter. I believe there's more of a spectrum of sexuality and who you're attracted to. I do believe some people fall somewhere in the middle. There is, however, a lot of stigmas placed on bisexual people and that they "need to just pick a gender". So I don't think it's that simple for them and in the eyes of someone who IS bisexual and is attracted to both genders, then yes, sexuality to them may seem like a choice.
When I was younger I was Bi-curious for the most part but as I got older I became exclusively attracted to guys.
But I always preferred guys even when I was young so maybe I was always gay.
