I'm not saying that it can't develop into something serious, but it typically doesn't. I hope to clear up some of confusion for once and for all. When you begin to casually date someone, you both need to know and understand what each other wants from it.
He always sticks them in the “hookup” category and gets on with his life of boning them and then having nothing else to do with them.
Imagine Greg waking up to 17 missed calls from that girl named Vanessa he took out to dinner twice; all because she saw him talking to another girl at a bar. In some cases, both of you might agree that you want to keep it casual but not see other people. But what if John and Becca haven't established if it's okay or not okay to see other people, then Becca gets asked out on a date? In Becca's case, she shouldn't say no on account of John.
Or imagine you're Vanessa, who thinks she just saw her new boyfriend Greg macking on another girl. Establish you are on the same page from the get-go. In other cases, you might agree that you both want to play the field and keep your options open. If Becca wants to say no, it should be for her own reasons.
You may develop feelings for them, but do they now just see you as a hookup and nothing else?
That seems to be the biggest problem in the dating world today.