I think a lot of people are fairly optimistic and trusting, and it often works out for us. We assume that when we buy food, it's not poisonous or adulterated with harmful things; we assume that our friends are not going to lie to us to our detriment; we assume that buildings aren't going to collapse while we're in them. A lot of those assumptions are based on the fact that it is against our wider interests to do wildly horrible, self-interested but other-harming things because if everyone descended into that behaviour, we'd be wiped out as a species. And those assumptions, and that reasoning, generally work for us.
And then we come to the lies that destroy personal trust. They are grim, are they not? We can meet someone and, not being wholly naive, take note of the fact that this person is generally honourable and trustworthy; has the trust and respect of others; is not - at least, not obviously - a complete sociopath. So, if they start telling us some story about their private life (which is hard to fact-check on) we may "verify" what they're telling us by way of checking out their other behaviour - which is generally supposed to be/checks out as trustworthy and good.
Hence the belief.
The questions, really, are why people wanting to conduct an affair will lie; do they believe it themselves; do they comprehend the levels of deception (and not just to their partners but also to the other person in the affair)?
I think they must believe it a bit (which will of course make them convincing) because it would take a conscience-less sociopath to behave in that way otherwise - and most are not sociopaths.
So, no, I don't think "these OW" are misogynistic, uncaring thieves of OPP, or secretly misogynistic. they are just people looking for a connection with another, for affection and love in an uncertain world, and think they have found it. Perhaps sometimes they have. Often, they end up very hurt, betrayed, and without a support network to support them.
I also think that we human beings really still have our "L" plates when it comes to understanding our nature, particular our emotional and pyschological nature. It's the only explanation I can come up with for the crazy, unkind and often quite self-delusional behaviour that people can get up to when it comes to love and relationships. The alternative is that there are a lot of Jekyll and Hyde weirdos out there - capable of being caring, loving people and then turning suddenly into completely, consciously sociopathic people. And I can't believe that. Though there are a fair few of those around