Sexual orientation is not an inborn characteristic.
*Puts on historian hat*
The modern concept of sexuality is just that - modern. It is only around 500 years old. We moderns tend to think of sexuality as a important marker of identity, as part of who makes us who are. I, for example, am a bisexual, American white woman. That's a very rough shorthand for important parts of my personal identity.
People who lived in Europe before the modern era did not think of themselves in this way. Sexual identity was not a component of personal identity in the same way we experience it. Someone similar to me who lived in England 700 years ago would not describe herself as straight or gay or bi. She would probably label herself as a wife, a widow, a daughter, or spinster, maybe add in where she lived in England, what her family did for a living (because she probably worked with her family). If she was Catholic or Protestant would have been important socially. She probably would not have described herself as white in the way I did because that racial category would have had little meaning for her. She might not have described herself as English at all - national identity was fuzzy 700 years ago - she might have said she was from Yorkshire. She also might have included her class - if she was wealthy, an aristocrat, or of middling rank or poor.
But not which gender she wanted to have sex with. This would not have been in her framework for describing who she was.
A man living in England 700 years ago would have considered himself manly if he had sex with women or men or boys younger than himself, especially if he was the one doing the penetrating. It did not make one gay to penetrate another man. It did make one effeminate if the man was being penetrated, particularly if no longer a youth but an adult man.
There are still many parts of the world where men have sex with men or boys and as long as they are the 'active' partner, they are not considered gay or effeminate. (No one seems to consider how woman would be 'active' in sex but that's a rant for another post.)
So until 500 or so years ago, there were no homosexuals. And no I am not parroting the right wing and saying homos don't exist - there were no heterosexuals until then either. (Oh, and don't confuse sexual acts for sexual identity. Just because someone has intercourse doesn't make them straight.)
Now personal identity is deeply rooted in us and that doesn't make it necessarily flexible. Although for some people, like me, it does. But just because someone has always identified as straight - or gay or flexible - does not mean that is a genetic condition. It's not. We are products of our culture.
TLDR: The idea that the gender which you prefer to have sex with matters in who you are personally is modern. Heterosexuality and homosexuality are modern concepts that have not always existed. They are not genetic conditions.
And, neither is poly. One is not born a poly or a mono or any other relationship variation.