I agree with Neuromancer, except that I believe the investment has to be an emotional attachment. It's not about getting caught, it's about violating her own values. And as for values, it's not about society saying extramarital sex is bad, it's about her betraying the man she cares about.
I'd say it's attraction + attachment. If she has no attachment, then she'll go for the first more attractive thing that comes along, regardless of what she promised. If there is some attachment, she will only go for others if the attractiveness is substantially higher. With a high level of attachment, no amount of attractiveness is a threat, not even movie stars or billionaires.
I disagree that high attractiveness (or in other words, continuing to do what you did to get her in the first place) by itself can keep a woman faithful. I think attachment is required, and attachment is a very different thing.
