It's fun!
Before my sister got married, she met my boyfriends really good friend and they started fucking. Just casually. She would share all the juicy details with me.
She ends up getting married eventually to someone else. She also eventually starts fucking my boyfriends friend again. I was always maybe a little jealous of the stuff she'd tell me about him. Now the dynamic is different. She's with someone and still getting that good dick on the side, I felt like I should too. I started fucking him too.
We both know. He knows we know as well. And no we don't get with him at the same time and won't ever.
With friends like that...