They live in a fantasy world and the "love" they are professing is not true love, but an infatuation/lust. Love to the mlcer is not the deep, wonderful love, it is having a f@ck buddy around for the convenience and excitement of the act...nothing more. The OM is nothing but a buddy and a band aid. Mlcers do not like to be alone, i.e., like teenagers always having someone hanging around w/them.
Sure, they can get over a 10 year marriage and breaking up a family very easily...why? Because the issues of childhood hit them very hard and come to forefront, whereby they stuff down the feelings that they have for their spouses and family. Depression plays a major role in mlc and if you read up on depression, you will see that depression comes in all shapes and sizes. Right now, your spouse is a teenager and has returned to that time to relive it because this is where she was emotionally stunted by someone in authority, it could be a parent, grandparent, teacher, etc., but it was someone of importance in her life that didn't validate and/or affirm her, i.e., didn't recognize her for her accomplshments. It could also be someone who may have abused her mentionally, emotionally or sexually.
Some day, she will begin to thaw her cold heart and soul and that's when she'll realize that she had it all and home is where the heart is. For now, she's out there cruising the world and you and the family are on the back burner. Sure, she'll peek out perodically, but it's to make sure that you are right where she left you pre-crisis. That's why it is important that you take the helm of your ship and start sailing your own course for now.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.