Not necessarily, it could just as easily be that he'd never even considered men as romantic/sexual partners before, but now he has, that's what he wants. Orientation is within the mind of the person, not an external assumption.
Willow Rosenberg is an example, she'd have classified herself as straight (and does) right up until the moment she meets Tara. From that point on she identifies exclusively as gay, never once bisexual. The fact she's HAD a sexual relationship with a male in the past doesn't automatically make her bi.