Getting back together with a cheating ex

My boyfriend of almost a year and a half was drinking at a town carnival and ended up having sex with another girl and texted me telling me I was going to hate him forever afterwards so he knew what he did was bad when he did it. Question is should I consider getting back together with him or not if he's never cheated on anyone including myself before until now and this wasn't an ongoing thing nor did they have contact with each other prior to hooking up?