There are several types of "other women".
1. There are the ones who never knew the man was supposedly in a monogamous relationship or possibly married. In her case she'd be as much of a victim as the girlfriend or wife.
2. There's also the "other woman" who knew his relationship/marital status before she got involved but didn't care because she was only out to have some FUN times. This happens quite frequently with men who are rich and famous. Everyone knows the guy is married and yet many women will throw themselves at these rock stars, actors, athletes, and politicians with the hopes of experiencing a fantasy or being a "Sugar Baby". They don't want to marry the guy or co-parent with his would be ex. It's all about money, good times, & sex.
3. Lastly there is the other woman who knew about the man's relationship/marital status and had no plans of going beyond "platonic friendship". They might be co-workers or associate in some other way. Their friendship was rationalized in their mind as : "Just because I'm married doesn't mean I can't have friends!" Over time they talked about various subjects and eventually covered each other's home life. The friendship gradually becomes an emotional attachment between people who "empathize" with one another. Initially they may have offered each other "advice" to better their respective marriage/relationship.
One day they give into their physical desire or one of them confesses he/she has fallen in love. Naturally this causes the other to confess their love as well.
In this scenario if the man is caught cheating that's when you'll hear him say; "I never (planned) for this happen". This is the type of "other woman" where a man might actually leave his wife or significant other for. It's got all of the "romantic storylines" of two people falling "in love" while belonging to others. "No one on the outside could understand us." blah blah blah
Odds are the wife/girlfriend could care less which type of "other woman" he's involved with unless she wants to stay with her man. Nevertheless it is the third type that is the most deadly as it was something that (evolved over time). This woman was not some human sex toy for him to secretly hookup with. He (cares) about her.