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.
If you are still sexual aroused/attracted by the opposite sex then Yes, you are straight. If you are sexually aroused/attracted to both, then you are Bisexual.
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.
You can tons of one night stands and still be straight. Sexuality is not relative unless you had it with someone of the same sex, so you're still straight.
