If -- as you say -- you don't want to be his girlfriend... then why bother worrying too much about this?..
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Either way, it won't lead to a relationship for you (because that's not what you want with him), so I think you should spend more time trying to figure out the feelings of guys you'd actually want to be involved with...

You're not confused, you're actually quite clear ... you "don't want to be his girlfriend".....
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What I'd say u are is ... curious ... curious as to what this guy's up to and seeing where this will lead to with u...
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If he's got a girlfriend and he's asking you on fishing and murder mystery dinner dates ... what IS he up to?..Could he wanting to be friends with you?..Do you want to be friends with him?..Why not ask him about his situation with his girlfriend?..Maybe they're not going out anymore and u just don't know that...
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Good Luck...
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Angie..
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I'm the yin to Carlos' yang .....

A year is an eternity, especially when you've only lived seventeen of them. I doubt that your guy would be able to tell you accurately how he'll feel next year, so why do you think we could predict such a thing? ..
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Have fun with this fellow, as a friend. If things change down the line, so be it -- you can deal with those changes as they occur. But for now, if the toaster ain't broke, don't fix it...
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Oh, and one more thing -- unless you have solid evidence that they're only faking it, give him and his girlfriend the respect of referring to her sans quotes. "Please."..

WTF? If you don't want to be this guy's girlfriend, then why are you so concerned with the girl that he's seeing, and whether or not he wants to get together with you, whether that's now or next year or ten years from now? Sounds like you guys are good friends, and if you're both happy with that, why worry about it? I wonder if there's something else going on underneath the surface of this question, like denial maybe...

Oh, please be careful if you really do want to be this guy's girlfriend. I was the sort of band-aid pretend trial girlfriend a time or two when I was much younger and guys I liked were dissatisfied with their relationships, and ended up getting really hurt. If you think he's not fully broken up with her, but he's flirty with you, he's still not broken up with her until you hear that he really is...