Thank you to all who posted while I was answering Daray.
I think it's helpful to me to see your assumptions that this could be a genuine mistake. I know that my two experiences (family of origin and marriage) with the closest possible relationships have left me highly cynical and suspicious.
I wouldn't call it a pattern at this point, but it's also not the first time he's said, "I misunderstood."
ETA: Paper Grace, he knows the whole history of the infidelity and lies from my xh. We've worked together for the last four years, during which time I was discovering more lies. Although I only made brief, broad statements at the time, he knew what was happening, and since we've started seeing each other, I've told him more about it.