I didn't make the choice directly. I fell in love with a man so I am currently with a man. I don't think of it as me too much choosing...I give equal genders equal chances...I think but I can be a bit bias since I do enjoy sex with a man so much more....there is a bit more of a fantasy that is filled out for me. I'm not saying a woman can't give it to me, I just haven't had that experience yet.
This may not be a satisfying answer, but here it is: you'll just KNOW. The first impression tells a lot; the first kiss tells a lot more. Trust your heart to tell you when you've found the right person. The only times I've gone astray were when I second-guessed myself. Listen to your heart and you'll find what you're looking for.
For me it's not really about gender, so regardless of whether my sexual orientation swings slightly more towards girls or slightly more towards boys, it still ultimately depends on how attracted I am to someone and how comfortable I feel with them and if I feel a relationship between us could work.
Like, I'm currently in a relationship with a man but if you asked me if in broad general terms I preferred men or women, I might lean slightly more towards women but that doesn't mean that I don't want to date my boyfriend or that I'm not attracted to him and possibly more happy with him than I am with a girl, I just happen to have found someone I enjoy being in a relationship with who is a boy.