How To Make It Work?

I spent the night at my best friend's. Her boyfriend came over and the three of us woke up together. I remember having sex with him, he then switched and had sex with her. We spent a lot of time just laying together on the bed, she and I were side by side holding hands. We fell asleep.

He has asked us to sleep together again. So far we have refused, sighting that we are uncomfortable. It's not true, we've talked and we are open to it, we just don't know how to make it more formal. We aren't going to let him get away with it and not have a commitment to us.

We also don't agree that we should have sex so he can watch. We are friends, if we wanted to be lovers we could have done that a long time ago. We are not lovers. But we are open to both of us being lovers with him.