I don't even think the most important thing here is a matter of objective reality on what each of them have to apologise for. I think the most important thing is their subjective reality and how much they *feel* they had to apologise for.
Not getting into the matter of the nature of the affair, they were both failing their marriage, unable to communicate and connect. And while I'm not blaming them because I understand the situation and how the trauma with Apocalypse's possession was too big for them to handle, they would - again - *feel* they let each other down.
EDIT: Also, there would be quite some ugly emotions there because, you know, they're both supposed to be human and pain and anger and trust and love and understanding become a big entangled mess in times like this.