How can I prevent Sex and how do I be strong?

Hi Im Dee Dee
I've been dating this guy for almost 3 months. I'm truly in love with him. There is two big problems: we are having sex and he is married. He's been separated from his wife for almost a year, and they are working on the divorce papers. At one point we tried to stop having sex and do things right but we just can't control it. He wants me to be his wife. I want him to be my husband. From all the guys I've been with he treats me right 100%. He wants to marry me in less than a year.We both want to do right but its VERY hard. His wife cheated on him three times plus has treated him wrong on numerous occasions. She has even told him that she doesn't love him and never shone him ANY affection. He's been with her for 14 years and has told me many times that I have done more for him than she ever did when they were together. Even his friends and family say that he seems more happier. I'm very happy, but at the same time I feel as if I'm missing something.... I need guidance.