Would you believe him?

What would you do if in an argument (because you half jokingly said he was flirting with a woman at work ), your husband said " I haven't cheated or tried to cheat on you since we've been married!" When you ask what exactly that means he refers to an incident that you already know about from when you first started dating in which he was messaging an old fling. The conversation went something like this:

Her: I see you and so and so are dating now.

Him: yeah but I don't think it's going to last very long.

Her: oh why not?

Him: I've been seeing her for like 3 months now but all of a sudden she is pulling away. it's starting to feel more like an expensive booty call.

I'm not sure how the whole conversation went, there were some hurtful things said and some compliments. I did pull away so I didn't really blame him, and didn't necessarily consider it "cheating", however I would bring up the expensive booty call thing every now and again in a joking manner.

Anyways my question is if he says that's what he meant and there was no other incidents, would you believe that was his only offense? Or would that get you thinking he may have cheated? It hurt my heart when he said that and I thought there might be a chance he did. Part of me believes him, however I have been deceived and hurt in the past so I do have some trust issues which are not his fault, but they make me more cautious and suspicious. What do you ladies think?