While men often get all the credit for being the ones to fantasize about having multiple partners at one time, women are also likely to have this common fantasy. Being involved with multiple people at once involves a whole mix of (again, totally normal) interests, like voyeurism (since you’re watching others), exhibitionism (since you’re being watched), curiosity (since you’re engaging with multiple partners and/or partners of the same sex), and simple excess, since if one person feels good, then more must feel better!
If you’re involved in a long-term relationship, this might be a secret fantasy you find yourself thinking of often, because it gives you various mental explorations and adventures that you might not experience otherwise, since you’re very much committed to the other person. While it doesn’t mean you want to have you cake and eat it too in real life, it simply means you like pushing the boundaries – at least in your head.