I've been the other woman and I think the relationship between him and his partner has usually broken down anyway. Something isn't working and the love has obviously gone.
While I'm so against betrayal I believe you are right.
I have an unusual take on cheating.
I believe strongly that the cheated on partner has fallen out of love with the cheater before the cheater cheats.
I believe that the most insecure people cheat and they are so paranoid about abandonment that THEY notice when their partner is no longer IN LOVE with them.
Being loved by their partner is not good enough for them.
They need evidence of IN LOVE as plain old LOVE doesn't cut it for them. They crave passion/romance.
I've shared my theory with several cheated on women. One said it made huge sense to her. Others don't grasp the in love bit. They say "But I loved him" not realising my point was "were you still in love with him?".
There's a big difference.
I believe too that it is possible to remain in love with someone. That it doesn't expire for all couples.
There's a graph about passion, commitment, love & friendship that psychologists use to measure the passionate ratio of a relationship.
If it reduces too much trouble looms.
I
Many years ago without my knowledge I was a 'side piece' for around two months until I found out he was shacked up with someone else. When confronted he explained that they had an unplanned child together and he hadn't wanted to settle down with her but felt he had to. He said he cared for her but wasn't in love with her. He managed to get those few sentences out before I got out of the car and walked away.
Ironically for a deceitful person I think he was telling the truth. He was literally no longer in love and neither was she.
The in love part had died so he went trawling for a replacement.
No excuses for his deceitfulness.