he will be upset and angry however you say it, because you are rejecting him. But he will get over it and hopefully see that you are right.
You simply have to say something like
'I want you to know that I have given this proposal some thought and not just dismissed it out of hand. However I can not agree to one last time with you for several reasons, the main one being that I do not want to do this.
We are not a couple any more and i feel that it is therefore not appropriate for us to have a sexual relationship. It's only natural for us both to have feelings for each other still, but doing this won't help us to come to terms with our situation as it is now and to move on.
More importantly we need to remain friends for the sake of the children and work together as their parents. I think one last time would lead to un-necessary complications and possible misunderstanings and prevent us from being able to work together effectively for the best interest of the children.
These are my feelings and I hope you will respect them and not try and get me to change my mind because I have made this decision and intend to stick with it.
You might disagree with my decision, but I hope that with time you will see it is for the best.'