Whether it's the safest thing as far as STDs is a bit beside the point. Even in cultures where the punishment for infidelity is lashes or death, people still do it. And, from the beginning of time, sexual monogamy almost never actually takes place, even if it is desirable in some ways - perhaps for the same reason that almost no animals practice true sexual monogamy - “A female who mates with several different males will have more genetically diverse offspring, boosting the chances that at least some of them will thrive.”

If you want to talk about safer sex practices, cheaters are much, much less likely to wear condoms or get tested than those who practice CNM - where both those things are pretty routine and expected.