If it's okay to be close friends with the opposite sex, why are they secretly sleeping with each other?

These people claim that it is okay to be close to the opposite sex, but many of them cheat on their partners with these close friends.

If it's “normal”, why can't they spend time with each other like “normal” friends?

If it's okay to be close friends with the opposite sex, why are they secretly sleeping with each other?
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