Why do guys keep "back pocket girls"?

I have a feeling this guy is using me as a back pocket girl.

One would think that the main reason a guy does this is to get some..but we have never slept together, he hasn't even mentioned it. The reason why I think he's just stringing me along is because he has not suggested commitment/exclusivity and calls me when it's convenient for him. If he's busy he couldn't care less where I am or how I've been. He's free Friday night? He'll call me and say 'let's go out nowww". I don't feel important to him, just feel like an option.

So he's not getting any sex from me, what does he want from me?!
Why do guys keep "back pocket girls"?
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