In small towns it happens all the time, and it's totally fine. I mean, just because you had a relationship with someone at some point in the past doesn't mean you own them, like, at all.
I had a girlfriend back in high school, we had fun, we went our ways, and a few months later one of my best friends asked me if it was ok if they could date. Of course, I said yes - they are now married, and might have never met if we didn`t go out in the first place. Life is bigger than your ego.