OM and OW just tend to be liars, pathalogical liars in most cases.
They lie to their partners, the om or ow, to their children and to themselves. They recreate justice to suit their needs and desires, affairs would be fine if complete honesty was divulged.
No one wants to be the person who is lied to, it's a cruelty and verging into sadism when it becomes a long term lie behind an unsuspecting person's back. To not recognise or acknowledge someone elses pain is in line with personality disorders.
As to your question aimed at ow, op, I would say although many of the deficits that make up someone's lack of concience are probably inate, nurture also has a bearing, but that can be linked with genetics also, having the same selfish characteristics, learning from parents, family and social circles that equate infedelity as the norm. There are actually great swathes of people that don't partake in affairs as recreational pastimes, maybe their lives have not been entertwined with such examples.
I also think it's a mixed bag of ow who have low self esteem and those that have an extra ordinary high level of self esteem.
I would say to replace a wife takes an extra ordinary ammount of confidence, much of which is usually attained by hiding behind the male, it takes a weird lack of insight or gall to brush off social disaproval of friends, family and especially children. I've seen ow who are totally detested by step children and hear them either ignor or minimise the situation, personally it wouldn't be for me.
To live a life, doing the right thing, being the right thing and not bulldozing over someonelses feelings for gain, is for many the right path, there are many things in life which you can't have without hurting others, but you certainly need a lack of concience, no doubt to partake in an affair.