You are desperate to marry him but he is not! This is because you have loved him truly but probably he has not. His secret relationship with other girl even when he was enjoying physically with you, relationship with her sister and his unwillingness to get married to you under various excuses; are straight indication of this. Better accept this bitter fact and leave him. It may be very traumatic for you at present but on long run it might be just better for you. Think over with these things in mind.