Some fundamental problems with your complaint: We don’t know where Ben is going to end up. We don’t know if he took the job offer. We don’t know if he’s going to stick around Pawnee or continue working in Washington.
TO ME, that scene was a man who saw a blossoming career ahead of him but didn’t want to lose the women he loves. The show, ever since the two got together, has been about how much they love each other. Like, it’s a sickening amount. They’ve beaten us over the head with it. It makes sense he would have a panicked engagement despite the obvious conflicts they face in the very immediate future.