If some one has made it clear to you that they have more to give, but they're not going to give it. What does that say? Why would you want to be with some one like that? I think you should have the self respect to take your talents elsewhere. That's bare minimum. Not everyone is going to be that honest and be that person where they say they are not looking for anything serious. Sometimes we simply feel that way as a result of their actions or lack thereof. And so i think that is the most obvious case of becoming some one else's back up plan when you try rationalize at and your oke with it.