You described yourself as attractive, honest and faithful woman. This show me right away you are a self confident woman that knows what she wants in life. Now your partner has two marriages that did not work out, he feels like he found someone who he loves, adores and he is so scared that he could lose the love of his life. He is nervous about who you are around thinking you might leave him for someone else. This is due to the fact that he does not feel good about himself.
You don't want to argue back or even deny when he gets jealous. You want to give him positive reinforcement. What you would do the next time he gets jealous is tell him that you are only interested in him because you love him more than he will ever know. He needs to feel self-confident in the fact that you love him.
He needs to be reassured because he unsure how you really feel. You could tell him you love him, but due to jealousy he is going to constantly be reminded how much you love and care for him.