her thinking he was cheating when really he was loyal apart from the fling with Rosa?
I'm not sure what your definition of loyal is.
By that token, was Yvonne faithful apart from the fling with Mark?
But anyway, I think the significance was that their lives were a little settled, dull; they'd been together a long time and sometimes people have a 'grass is greener' moment. And that life/marriage/morality is complex –Yvonne couldn't really take him to task about Rosa because she was also guilty of an affair, and vice versa with Mark.
One thing that left a slight bad taste in my mouth was that Mark 'got away' with his fling with not much in the way of consequences, whereas Yvonne's was 'punished' by a rape, stalking and ending up on trial for murder.