Everyone is different, someone could be married for years then marriage breaks down be it down to money, stress of work, teenage children causing problems so if one partner met someone else I would understand it more unless the person went out and looked for someone to cheat with. Men normally get the bad end of the stick here as women have more sympathy when in a relationship that breaks down, men are automatically seen as cheats and will cheat again when women cheating on a man get the sympathy that a man wasnt good enough, he had no respect for her etc.
I had a ex girlfriend treat me like dirt, I am shy so was set up with this girl who sent me over 300 texts in 3 days, 500 texts the week after and told me I was using her if I didnt respond instantly, she then went online and told guys either she was single or I led her on and didnt respect her getting guys saying they would date her and they bought her little presents like paying for her phone bills or a new games console, we lasted 3 months but I hated every moment, she even had her mum fooled as her mum said I was scum and leading on her darling girl.
Love is a complicated thing, and easy to misunderstood affection and companionship for actual love thats why many modern marriages fail, or you can love things about a person like their personality, apperance, but to live with them is a different experience as they could be a different person at home.
I would understand falling out of love more than someone flirting with some randomer or even a friend as thats being unfaithful to me(not cheating though) since its a betrayal of trust rather than just being unaware of whats happening and falling in love with someone without even knowing it.