It's possible they were always bisexual, it's also possible that changing hormones could change people's sexuality. We don't even really know what causes people to be homosexual, its entirely possible that introducing other hormones could cause it to change. This would not make it a choice. They didn't choose to change their sexuality it was a result of the chang in hormones effecting their body.

Or like other people said they could be lying, but given the significant lack of research in this area I'm not going to pretend to know whether it is or isn't possible.