3rd party only wants to be with my girl
So my girlfriend and I have been doing very well! We even met a guy the other night that I surprisingly REALLY liked! She likes him alot too! I've never been a "man-hater" or anything like that. I just never really connected with guys, but he was WAY different! I'll call him J. My girl and J just met about a week ago, and then I met him a few days after so it's all still very new. I felt very comfortable around him in a rather, intimate way. Which was strange to me. All 3 of us had a great time!
Now, my girl and I also have another guy named, T. T is dating my girl but not me; he and I just aren't into eachother that way and that is okay with all of us. My girl has explained to T that she wants a gf (most likely me; her and I are just taking it as it is) and to be able to play around with other guys; I know that I want another girl in my life and possibly a guy. T has said... he only wants to be with my girl.
Now, I think T is a great guy, I really like him, I have rarely felt jealousy towards him (and when I did I just analyzed my feelings and found the real point of it) and I know my girl loves him. But what T said about *only* being with her worries me. I personally don't think it's possible for a couple where 1 is mono and the other is poly to be together; it just seems very unhealthy to me. It's like a gay person and straight person being together; it is possible for those two types to fall in love with eachother and truly care about one another, but that one SMALL detail (gay vs. straight / poly vs. mono) can make a HUGE difference.
Am I being out of line or unreasonable? Does this happen AND work out?