When engaging in any ritual, the participants in that ritual may or may not be fully engaged. Some go through the motions because that is what is expected of them by those around them, especially when that expectation comes from the leaders or priests. They do what they are told is expected of them by the gods. They thought they were pleasing the pagan fertility gods. That is very different from an enduring pattern of emotional and physical attraction.<quoted text>
But repeated sex acts do define a "favoured" sexual orientation.
By your logic, you would have to assume each male or female that was a heterosexual, was having a single "bi" experience and never again, leaving them to be a heterosexual.
So Paul would have been speaking of "first timers" and "repeat partakers" and the "repeat partakers" by their actions, would define themselves as bisexuals since they were enjoying repeated sex with females and males.
You are using your modern, Christian societal perspective, rather than the pre-Christian pagan society perspective that Paul was living in and writing about.