The problem I see with this question is that it's not one that's been PROVEN scientifically one way or the other-therefore there is no way to give you a FOR CERTAIN answer. Only opinions.
In my opinion the answer is-both.
For some people it is simply a natural part of who they are like having blue eyes, just like Mono on here is mono, it's a natural part of who he is like his height.
However-for others they are innately
CAPABLE of BEING IN LOVE WITH AND ACTIVELY LOVING
more than one person at a time which is polyamorous. This is a natural part of who they are like me having blue eyes.
THEN there are people who live a lifestyle because they choose to, it's not natural to them, but it's more comfortable for other reasons and they choose it.
Like Maca, my husband. He's naturally polyamorous-but he always lived mono because bucking the system was too emotionally straining for him to bother with.
Additionally there are people who choose to live in a poly relationship because they find it easier than trying to live mono even if they are more naturally mono.
NO ONE on this board knows what the situation is for your gf. Only she does.
THE REAL question is-can you ACCEPT HER AS SHE IS.
Because in ANY situation-the only way it works is if we accept the other person for who they are RIGHT NOW.
"Love As Thou Wilt"