Mmm. My theories (I tried to sound the least insensitive I could pls don't jump at me
):
Yes (Born):
-If it's a mental disfunction as a consequence of a wrong brain development.
-If it's due to a genetical disorder involving a hormone-related gene (likely imo).
No (Environmental):
-If it's consequence of your life experiences, your environment, etc.
Yes and No (Born+Environmental):
-As someone else pointed out, maybe we are all born bisexual and environment just makes us like one thing or the other more. (most likely imo, that's why I voted Yes and No
)
In any case. it's not natural for a species so sadly there must be a cause for it. It can't be random...