In a 2007 article (now in his website archives), atheist Richard Dawkins frowned upon those valuing faithfulness. In “Banishing the Green-Eyed Monster,” he asked, “Why are we so obsessed with monogamous fidelity in the first place?” He says that we need to intellectually “rise above” the sexual jealousy that blind Darwinian evolution has produced in us and cheerfully permit our spouses to carry on any sexual recreation they choose to have outside the marriage bed. “Why should you deny your loved one the pleasure of sexual encounters with others if he or she is that way inclined?” he asks. Dawkins is being completely consistent with his worldview by denying any objective sexual morality, even where marriage is concerned, so long as no one is harmed. (It remains unclear, however, why harming anyone or anything is objectively wrong in his view, where morality is nothing but an evolutionary social construct.)
My sense is that most self-described atheists, particularly those who are in happy marriages, have a deep-seated conviction that something is truly wrong with marital infidelity. As God’s image-bearers, the Truth of Things resonates strongly within their soul, and they know that spouses should be faithful. Just like they know, intuitively, that it is wrong to stab puppies with scissors for fun. The Apostle Paul talked about how even those who do not profess to follow God often still follow His law, for it is written on their hearts—“their conscience bearing witness and their thoughts alternately accusing or else defending them” (Romans 2:15).