I became predominantly attracted to my own gender a year before high school. My attraction to the opposite sex diminished to almost nothing in a very short period of time while my attraction to other guys greatly increased. In my childhood, I had lots of crushes on various girls, as well as immature sexual fantasies. After I started going through puberty, I developed some sexual attraction to guys, but in mostly a non-emotional sense. During that time, as with most guys at that stage of development, erections happen frequently, and I found it incredibly easy to get aroused by lots of different things.
About half-way through high school, I started having crushes or romantic feelings towards guys that I liked, whereas before that my attractions were mainly just physical or sexual in nature.
How and why I lost nearly all of my interest in the opposite sex, I have no definite answers. All I know is that it happened and the process went very quickly. I would say that it happened in less than half a year.
So in all honesty, in my case, I cannot say I was born that way. For me, it was something that just happened rather suddenly.
I know other people have said they felt those attractions in early childhood too, and I believe them, so I am not able to judge the situation for them. I only know what I experienced, and I am forced to conclude that it is a combination of factors, and that for some, different factors are more prominent.
I can't see how anyone could pick one simple reason for sexual preferences.