To be fair people can justify a lot of things when they fall in love with someone. It was his marriage to be faithful to and his duty to his wife to not cheat on her.
And being prepared to be complicit in his cheating makes her a selfish slapper who doesn't care about his wife and kids getting hurt.
TBH I get sick of hearing the 'you can't help who you fall in love with' bullsh*t. Maybe you can't but you can
help what you do about it and act like a responsible human adult to avoid the consequences of your own selfish sense of entitlement on any children involved.