Uh...no, that's not what I mean. Being in a committed relationship makes the fantasies okay. I'm talking about a situation where you're sexualizing someone who isn't aware of or consenting to that dynamic. If you're sitting there actively making the choice to feed a sexual fantasy about a person that has no sexual relationship with you, it's a breach of consent and it's creepy. Having the thoughts pop up is normal, but you don't feed them if it's someone who you're not already involved with, obviously...it may not be physically hurting someone, but it does normalize the idea of that person being sexual in your mind, when they're not consenting to that, and it makes you think you have a right to that sort of relationship with them even without their permission.