I've been seeing this happen more and more lately. It's not a healthy mindset to be jealous about it, especially if all you saw her as is a jumpoff.
Don't get me wrong, I get it. I always reminisce briefly when I see a girl that I used to happily drill away at in college or in law school get wifed up and start out on her track to getting a suburban house, kids, and a picket fence.
It just happens.
The healthy mindset that I've found works for me is to look at her as someone that I had a lot of fun getting to know. Someone that I had some awesome drunken fun times with and had some fun marathon sex with. Someone that you shared an experience with in college knowing that it wasn't going to pan out to anything majorly serious, but you still can leave each other behind with some positive memories and be respectful.
Again, don't get me wrong, I know it's weird and it sucks because there's a part of you that wishes that you'd have her availability for that kind of fun forever and ever, but if you look at it less as a territoriality thing and more about some fun you both had together when you were just kids, it's easier to mentally embrace this kind of thing.
In short be happy for her, be happy for the guy, go outside, and make more memories.