No, not necessarily. Having fantasies that include people of the same sex are not sufficient in determining someone’s sexual orientation. A fantasy is part of one’s imagination and is often not realized. The desire to live out one’s fantasies, on the other hand, can offer greater clarity as to sexual attraction. It’s important to remember that emotional attraction also plays a role in determining sexual orientation.

Listening to your body and understanding your feelings will gradually allow you to determine your sexual orientation.

Sources:

Dictionnaire Larousse. Définition de fantasme

Trevor Project