I feel bad, but I also think it's necessary! Buck really needs closure, and Abby having a fiancé, and not just a boyfriend, is really the ultimate way of showing that Abby has moved on, and it's okay for Buck to go and be happy too. It's some closure he really needed, and I think we needed too as an audience.
It sucks though, and I imagine that's how it's supposed to be. This sucks, but also it happens. He'd already moved on, but now he hopefully won't hold back. He already has other great things in his life :')