Why would he choose her over me?

So the short of it is this---I like a guy, we flirted, we talked, and I was sure he was going to ask me out, but then this other girl started talking to him and now it appears they are getting "involved". The thing is that this girl is a complete slut and in fact just got out of a relationship. It's so frustrating because I have liked this guy for a long time and have been talking to him for much longer, yet, she came in and is now gonna get him. =(

I do not mean to make it out like I am better then her, but it just seems so obvious that she is not interested in the things that make up a mature relationship which I thought he wanted.

So guys, why would a guy chose the slut over the genuine, wholesome girl? I mean besides the "easiness" factor of the slut---or is that the only reason?
Why would he choose her over me?
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