A Shaky Foundation???

I have a lot to say but very little desire to share it all, but here I go.
I spent my whole weekend with Apple Jacks and it was great. I didn't want it to end. We did face one hick-up.
We are walking down the street wait for a table for Sunday brunch... I had my fingers hooked in his back pocket. La-de-da walkity walkity. I hear foot steps coming up behind and I slowly menuver Apple Jacks and I over to make room for this faster walker. All of the sudden I hear, "Apple Jacks!" We both look and there to our left is this dark hair woman. Apple Jacks and her chatted about how she got a new car or something and Apple Jacks seemed to be a little uncomfotable. I just watched. This chick starts to walk away and she turns over her shoulder and says something about introductions. Apple Jacks blushes, tries to get the chick to stop walking and do introductions. She finally turns all the way around, still walking, and says "It should just be said he's dating multiple people." and storms off.
I give Apple Jacks a look and start laughing. This had to be the most spiteful, bitter comment. She was oviously hurt by him and wanted to spit it in his face, and my face. What still astounds me is I didn't flinch. There is no part of me that would think Apple was seeing other people. We are so comfortable and so intence I don't think it would be posable... who could have the energy?
What was the worst part was, though none of this got into my head, it did get into Apple Jacks. He felt like an ass. He totally explained himself. He was dating her (3 dates) and meet me and stopped "thinking" about her (I think there was a date or two overlap). So, he didn't call or e-mail her back. He admits this was a shitty move and the wrong choice, hence her getting in his head.
Later that day he got an e-mail from the dark horse and felt the need to e-mail her back. The next day he called her to "close the deal" where she eventually started the "Why aren't I good enough?" and "Why do I only date people for three dates? What's wrong with me?" Which solitified any doubt he might have had that he was doing the wrong thing.
But here's the deal... My biggest issues to start with when it come to any sort of relationship, busness, friend, more... is TRUST. Here is a big trust issues and I'm only slightly worried. He has given me, to my knowladge, full disclousour. And I know only time will tell and only time can build trust. So... I don't know.... I guess i'm hanging on and doing my best to build a solid foundation.