I can think of two reasons. 1) When you two are together swapping with other couples, there are natural boundaries and ground rules. It is play. Emotion can be left out of it. There are other people around to keep things from getting violent or coercive. If she is alone with another man those guardrails are removed. She must have a greater amount of trust in her playmate and that risks emotional involvement, which she may not want. 2) If she plays without you, then she is implicitly giving you permission to do the same. She may not want to risk you becoming emotionally involved with another woman. Whether or not that is your intent, you need to be clear with her and have an understanding.