Artemis welcome to the board you will get some really good advice here. I am going to refrain from giving you advice right now because you are new on the board, but I will answer your question and comment below.

Originally Posted by Artemis1
Should I be having sex and sleeping with him?

Absolutely not! This is known as cake eating.
Originally Posted by Artemis1
Also almost every day he has told me he loves me and that I'm his Best Friend still.

In these situations you need to look at actions only and not words. Do affairs and talking to other woman (actions) support the words?