Continually tell her how much you love her approve of her ‘spreading’ her wings sexually. Start discussing scenarios that will allow her to visualize and get comfortable with being with another man. Do things like bring home a lingerie catalog and pick out items that you think she would look hot in and leave it where she will see it with notations detailing how hot it would be to see a guy take it off her or fuck her in it.
Remember, she loves you and does not want to risk losing your esteem, love, or approval.