I don't think deciding to stay after an affair or whilst another one is going on is that unusual, I know of several couples of varying ages and stages of life that have done this, what is more unusual is to be very happy about it and want it to continue.
Most cases I know are where the wife doesn't ask questions because they simply don't want to know the answers so don't ask them or don't look too closely at suspicious behaviour.
I also know at least three families where the husband had a long-term mistress including in two cases additional children but again, the original wife chose to keep the marriage, sometimes right up to death, rather than confront the actual situation.
I also have a couple of friends that know about or suspect their husbands (multiple as it is rarely about one person) infidelity, but they have not worked or work in p/t jobs and having seen their likely lifestyle, and the number of their kids (three in both cases) have chosen, possibly wisely IMO, to stay with them for financial and ease of life situations. They don't want to be single parents or on benefits or running round endlessly driving the kids everywhere which is what would await them if they jumped in mid-life and got a divorce.
None of this is for me, I love to be the centre of my husband's world and so I could never begin to cope with any competition whatsoever, but I can also see that there are many living situations that are far more complex than what is portrayed on here (romantic love vs financial arrangement).
Occasionally the wives have returned the favour by having a relationship of their own, although this tends to be short-lived and more like a revenge thing from what I can see.
I don't judge this at all, but I think the chances of the boat not rocking or your children being affected are nil. I hated my dad having constant affairs, it's very destroying to your peace of mind as a child.