To give context, the both of them still live together but broke up 6 months ago. My friend and I met by hooking up before they started dating who he also met by hooking up. The three of us had gotten close during the time that they were dating (they dated for 1.5 years), but after the breakup, she distanced herself from us, rightfully so. To clarify it was mostly head, and we debated it for an hour. Before it happened, we did also ask her if she was fine with it, to which she said yes. Granted we did still know it was wrong. To make things worse, I had just had a conversation with her about how I would be there for her. Honestly, we did it partially out of spite, in the past 6 months we did our best to include her in plans, yet she kept saying she felt excluded and left out when she was not making an effort to stay friends. I understand that fundamentally it was wrong, but to be fair we did get her permission, and it was purely sexual with no romantic feelings. We are whores. And she knows it. How do I go about apologizing, and how bad is it really?