I might get berated for this, so everyone just hear me out. I believe that sexual orientation (whether straight or gay) is a subconscious choice. The only evidence I have for this, happens to be that empirical data has proven that people are attracted to people who in one way or another remind them of someone they have a lifetime bond with (mother, sister, family friend, aunt, etc.) So, if one is attracted to people that somehow relates to their previous life experiences, then isn't that who you are more sexually oriented too (i.e. maybe a person is more oriented towards brunettes with large smiles because his mom and sister are that way, or a person could be more oriented to a person with a quirkier sense of humor, etc.) Therefore, I feel ultimately, your environment shapes your sexual orientation.