The way Stephen says it strongly implies that Jade knows Stephen didn't fuck Bella. I take that to mean Jade and Stephen approached Bella for some swinging fun and Bella shot them down with her trademark death glare.
Combine that with the "Has he forgiven me?/I have forgiven him" lines, and my take is that Bella and James had an open marriage of some sort, but one of their trysts went spectacularly wrong and effectively broken them up. Bella, who was quite the player up to that point, blames herself for it. So she has tried to be a traditional, faithful wife ever since.