People swing in order to fulfill unmet sexual needs. This practice is considered as one of the ways to reduce prostitution since it happens in the open and most swing parties involve couples only.
However, religion especially Christianity is strictly against polygamy at all costs. From the Christian point of view, a man should be committed to only one partner, and so should the wife be. Sex outside the matrimonial bed is considered adultery, which is a sin according to the Holy book.
In addition, this trend has shattered many marriages. While most couples participate in swing parties to save their marriages, they often end up leaving their partners for good. Once you have a sexual encounter with another person, it is hard to stop.
Most swingers continue seeing the people they slept with even after the swinging excitement is over. They keep going back for more especially if they found the other person more satisfying than their partners, and so they end up sustaining an affair with the swinger. This infidelity leads to broken marriages as couples try to find satisfaction outside marriage.