It has been my experience with online dating that men immediately begin using a pet name like sweetie, cutie, or even sexy. Why is that? I don't know about other women but I really like it when a guy uses my name. That makes me feel acknowledged and special. A pet name can come later when we really get to know each other.
I think a lot of men aren't aware it pisses you girls off and it's more or less their way of verbally admiring you in a seemingly innocent (or maybe less innocent) manner.