What would you do?

So I found out last night that my partner of 8 years had cheated on me two years ago. He had kissed this woman before and promised me it was just the once but this is a separate occasion with the same woman. As far as I am aware it’s only supposed to have been a kiss but I find this hard to believe if he didn’t just tell me. Currently pregnant with his second child and have a 6 year old xx