IMHO there are two main flaws with what you say lostdomain and I also think the gender issues are interesting.
Firstly, I've never said (or thought) that boredom in a relationship is particularly a motivating factor in these types of affairs. Familiarity possibly, complacency definitely, but escapism from life itself (not necessarily the relationship) is probably nearer the mark. I also think the chronology is a bit wrong in what you say. It's more a case of: an opportunity presenting but not being seen as one, a connection being made that's viewed as perfectly safe, poor boundaries and then some realisation which is often unconscious that the new connection has added spice and drama to a life that was previously quite content, if not full of excitement and adrenaline all the time. This is when the addiction to that adrenaline and new life force takes hold and the rest, as they say, is history.
It's a very lazy kind of adrenaline fix too. Organising extreme sports, changing careers and going on world trips takes considerable effort and money. Starting an affair with someone at work where you have to be every day and even get paid for it is a low-effort and completely free adrenaline boost.
From what I've seen, these affairs are commonly entered into by people who typically need 'props' to get through life. They manage okay when life is going smoothly, but as soon as something happens to alter their equilibrium, they reach out for a familiar soother. The more inherently lazy and selfish the person, the more likely they will reach for a quick, lazy and cheap fix. The equilibrium distorters can be incredibly varied: getting older, altered appearance, new role or job, loss of an old crutch or prop, loss of an important life role, failure, increased stress and responsibilities.
If there's vengeance involved, I think it's rarely (in this type of affair) vengeance against an existing partner or children. Vengeance against a life that's suddenly got a bit difficult? Yes maybe.
I think you also might be applying a rationality and logic to people's decision-making that unfortunately goes out the window when lust and admiration take over. Lust and romance are incredibly powerful motivators and as long as children especially are safe and well and not apparently directly threatened, people believe that it's not as stark a choice as choosing sex over the welfare of children. People like the OP in fact kid themselves that they are better mothers or fathers because they are happy and that on the contrary, the extra sex and romance enables better parenthood. The realities of mental absenteeism, the lows that separation induce (especially at christmas) not to mention the extra hours spent after work or leave days spent on the affair, get erased.
That said, I do think there's far more judgement about mothers having 'bit of fun' affairs, which is ironic seeing as women who have them are more unlikely to lose residence with their children than a father in the same position- and so their risks are fewer. I also think people are more morally judgemental of women seeking extra sex, than men.
My view is also that it's an old-fashioned concept that people's partner relationships define them so much that any self-inflicted attack on those relationships is thought to be a signal of relationship discontent. We don't apply the same judgements to other 'props' such as alcoholism, drug addiction, compulsive gambling or shopping addictions. When those occur, we tend to look at the addict's life in the round; their working lives, their friendships, their extended family relationships, their interests- as well as their partner relationship.
I'm interested therefore in what this OP was trying to escape when she got involved in her affair and why she felt she needed a prop to get through life at that point. I hope she considers that even if she won't speak to us about it.