It WAS a marriage, marriages are works in progress... ALWAYS...
A marriage is a boat, you can help steer, paddle, row, etc, or you can sneak into the hull and start digging a HOLE in the bottom and dance with glee that eventually the boat will sink and everyone with it.
She cannot justify an affair by waving her hand at her marriage and sayign "that's not a marriage"... Her AFFAIR isn't a romantic relatinosihp either, its a sleazy affair based on lies and hurtfulness...
Comparing an affair to a romantic relatinship is akin to a Stalker comparing his obsession to being "in love"
Both the affair partners, AND stalkers are DELUDED.. its an unhealthy attachment to a fantasy... it will fail.
I would take yourself out of your girlfriend's facebook accounts so they don't have a lifeboat to turn to when their marriage starts giong south... and they all do run into bad weather...
What are YOU going to do when they contact you to "talk" about their marital problems?