Thanks for submitting your question to Alterheros! There could be a lot of different reasons as to why you are fantasizing about other women while with your male partner. I think its important to realize first off, that your sexual orientation does not necessarily come with a set sexual behavior and a set object of sexual desire. For example, you can be with a male partner, but you might not always think about him to become aroused – and that is perfectly fine.
Fantasies are called fantasies for a reason. They only exist in our dreams and thoughts! It is perfectly normal and common to fantasize about a sexual act that you might have no desire to actually perform in real life. Now, this doesn’t mean you are mentally cheating on your partner, or that your sexual fantasy is a reflection of what or whom you desire sexually in real life. Sometimes, people act out fantasies and realize that it went way past their comfort zone and that it is sexually stimulating as a fantasy only.
On the other hand, sometimes people act out fantasies with their partner, and find it extremely satisfying and a new fresh mix into their sex lives- and that’s okay too! Important note: discussing the fantasy prior to playing it out with your partner is a must!! If you do not discuss it with your partner beforehand, they may react unhappily or with a lot of discomfort which would not bode well for the health of your relationship.
Another possibility is that you are interested in women as well. There are many people who find they are attracted to women and men, they might identify as bisexual, but they do not necessarily have to incorporate this into their sexual identity. Your sexual behavior and object of sexual desire do not have to define your sexual orientation or sexual identity. Its important to remember that regardless of whether you are with a woman or a man in a relationship, dating, or a just a casual encounter, your first priority is you! Make sure you are in a safe, consensual, and healthy situation. That means the other person knows your boundaries, cares about your welfare, and that you are practicing safe sex. Also, of course, make sure you have fun! = )
Only you can decide what the nature of your fantasies are by further exploring them. I hope that has helped clear some things up for you, and has given you some things to think about. If you have any more questions, please feel free to ask!