Found out partner had slept with one of close friends in the past and lied about it

So bit of background - my partner (32F) and I (32M) have been together nearly 4 years now and we initially met each other at marketing events over the years,work in marketing, and eventually became introduced through some mutual friends.

One thing that I knew about my partner was that she was close friends with a male friend of ours, let's call him Jake, and she would regularly catch with him, alone, for drinks or for a chat and the occasional "D&M". Now I was under the impression they were just really good friends as they'd known each other for several years, so I had no issue with them meeting up.

Now fast forward 3 years, we're not as good friends as we used to be with Jake, and my partner decides that she wants to confess something to me - after the end of her long term relationship and the months leading up to our relationship, she had slept with Jake several times (I should also note, he's known for sleeping with many of the girls he hangs out with) and they both decided together not to ever mention it to me after we had started our relationship.

Now look, I don't have an issue with not mentioning certain things about your past (everyone does it), but when it involves someone which we are seeing on a regular basis, and someone which she chose to continue hanging out with one-on-one, I feel I'm right to have an issue with it? Because when I look back, it completely changes the dynamic of the situation in my head and becomes something that I would not be okay with if I had known the full story - because it changes it from "just friends" to two people who find each other attractive and have acted on that in the past. Now I don't think anything ever happened while we were together, I'm not worried about that, but I don't really know how to handle this going forward because it now makes me question if there's anything else she hasn't told me.

Has anyone else ever been in a similar situation or can offer any advice? Do I just forgive and forget and take her promise that nothing like this will happen again?