It has nothing to do with expecting her to be on a guilt trip for the rest of her life. You fail to see any mixed messages, others fail to see how there can be anything but. Friendship yes, relationship no, if they feel that strongly about one another get married. They clearly don't feel that strongly, they just want to play at it all! Would you think it's OK for Sarah to have stayed married to Andrew and continued with her lover, because she felt strongly about that realtionship and couldn't end it?
If they were that concerned for the girls, Sarah wouldn't have gone in for a bit of toe sucking or Andrew would have stood by her when she was outed!
As I said, friendship after a divorce, I'm all for it, invite her and her partner to stay, fantastic, announce to the world in general that you live as a couple, tacky. I wonder how Andrew feels about yet another tell a bit interview with his ex wife.
So after they marry and possibly divorce, because, well we can still live together 'as a couple', you will still think it's OK. I don't believe I have called or inferred it is an abomination, the label I put on it is tacky!