i feel you determine it just having a fantasy doesnt make you anything always having straigt/gay fantasies may be an indicator
I don't think so. I think they can be indicators for some people, but that it's normal to fantasize about things you might not be comfortable doing in real life. It really depends on how you interpret them and how important they feel to you as there can be a lot of different causes for fantasies, and it's how you feel about your orientation that is the most important.
No fantasies don't determine or create your sexual identity, but they are a good indicator and if you are having certain fantasies frequently it is probable that they are showing your identity.