Side guy

So there's this guy that I've been messing around with for months now outside of my relationship. Recently my BF proposed to me and we're moving in together. So I told the guy that I can't see him anymore. Now he has a GF as well and when I told him the news he was acting all upset and saying that I was hurting him. I wanted to see him again but the timing is just off. What's your opinion on this situation?