Swinging will not increase the chances of your partner leaving you, in fact by being open and honest with each other about your sexual needs and fantasies, and working with your partner to explore all your fantasies, you will decrease the chances of them looking for sexual releases elsewhere.
Sex is not usually the major driving factor in relationship breakdown. It is more often a failure to communicate.
Swinging should not be used to try and save a relationship. If you are already in a rocky relationship then swinging will not help to bring you back together.