Why is it, do you think, that you are you gay? (SERIOUS)

My girlfriend has a belief that people aren't born gay. I disagree. I do believe that some people can be bi and then it comes down to preference.She feels that how people are raised and what they experience shape who they become.Ex 1: Guy raised with 3 sisters, mom and dad. Later comes out (not to my surprise at all as I saw it from a young boy in him), she feels perhaps it was because of so much woman prevalence. I saw evidence (nothing sexual, but the mannerisms of effeminate men that are stereotypical gay), much before he was old enough to be "playing it up" if some feel that is really a thing.Eg #2: She feels some women chose to be lesbians as they are fed up with men and being mistreated. My thoughts are that the only women who would do that is those who are bi, or didn't come out as gay until having to try and pretend to be straight.Reason I feel this way is because no matter how hurt or pissed off I was I would never suck another dick as my only viable option. I can't "turn gay" because I am sick of women's bs and I don't believe women do this. I have had a gf that was with a lesbian just prior to me, and that woman was a lesbian, my gf at the time; was bi.My sister was with guys growing up and then a few years after she had a little guy she came out as gay. Now I am in no place to make these calls, but I feel there are surrounding factors in which she could be bi and now have the resolve to focus solely on women. But all the women she dated were 0 penis allowed ever, lesbian.BTW this is incredibly insensitive but forgive me as I need something to gain perspective for either her or myself or both. I feel I look at it from logic only but she has faith to contend with too.There was a story of someone giving testimony about repressing those urges, to which I said "Are they happy, or just living to please others and their expectations".Don't get me wrong, we both have 0 against gays, but I would love some tangible text to see if you feel something being raised edged you towards being gay, or if you were born that way.Please if you could denote gay or bi so that we can know your stance. I am just trying to expel ignorance in both of us. Thank you for your help.