When you fantasize about having sex, if your imagined object of affection is of the opposite gender, you are still heterosexual. If you are still attracted to others of the opposite gender, you are still heterosexual.
Everyone is different. My definition of sexual orientation is 'which gender will a person yearn for once he/she knows everything? ' In other words, it takes years and years to really know oneself. No rush. Enjoy the process. It's dating. Dating should be fun.