If you have truly healed the wounds of the past, then there is no reason to avoid something you both find enjoyable. Simply make clear between you that the swinging will be about doing it together and sharing the experiences. Sometimes surviving the hurtfulness of cheating makes a couple stronger and imbued with a much healthier perspective on what's important in a marriage. That alone can make the joys of swinging available without hurting the relationship and with the possibility of continuing to strengthen it through shared pleasures AND JOYS.