The fact that infidelity occurs in all sorts of relationships, is practised by all sorts of people and occurs at all sorts of different life points, means that there are no "types".
There are just vulnerabilities.
None of us can say if our partners would be tempted, because we are not them. We can only summise what we would do and hope that we are right. We can only ever control our own fidelity, not someone else's.
I've seen your posts about this before OP and you still seem to be focusing on your attractiveness quotient, when in reality that means absolutely nothing. I also still think you suspect that a great relationship is the only insurance policy you need. It isn't.
That said, an egalitarian relationship characterised by equal, mutual investment is the least vulnerable, in relational terms. But often, even that is not enough. The most vulnerable relationships are those where an unfaithful partner doesn't maintain his investment in the relationship/family and infidelity can become very gendered, with female SAHPs married to selfish, under-invested men, amongst the most vulnerable relationships there are.
I also agree with Gay that no-one goes into an affair against their will and that the conversation she overheard was the man's signal that he was open to an alternative relationship.
I have read a lot of research too about how there will always be weak moments in relationships and I think that's realistic. I've yet to find one LTR that didn't go through a tricky patch, or the individuals within that relationship, a personal low-point.
I imagine we all recognise that in every marriage or relationship, there will be those vulnerable times (pregnancy, the stress of young DCs, bereavement, job /esteem losses, mid-life reckonings or relational discord) and there are some OW/OM only too happy to exploit that set of circumstances and offer a diversion. But as ever, it's up to the object of these attentions to say "no".
It's just that sometimes as humans we're in a stronger place to say that, and at other times we're not. That doesn't make us evil or bad; just human beings with flaws and vulnerabilities.
I don't think it helps to demonise infidelity, because that adds to the erroneous belief that it can never happen to you. All any of us can do is work on our own vulnerabilities and set clear boundaries with our partners, discussing fidelity in realistic terms and not just in relation to the penalties that will occur if there is a transgression.
Better to assume that this could well happen and talk honestly as a couple about how to guard against it.