Office settings are hotbeds of adulterous relationships, driven largely by the fact that the people you work with are often the people you spend the most time with, including your spouse.
An article in Newsweek stated that infidelity in the office may also be on the rise simply because women are establishing more powerful roles in workplaces than they were 40 years ago. The authors noted that nearly 60 percent of American women now work outside of home, a jump of 20 percent since 1964. They are also reaching higher positions of power, meaning there's simply more interaction between men and women at work than ever before.
While office romances are widely considered to be unhelpful to a professional environment, there isn't really anything companies can do to stop them. Superior-subordinate relationships are understandably looked down upon, given the conflict of interest that arises, but explicitly banning adultery is impossible for a company to do.
Ultimately, though, it's probably best to avoid getting into an office romance, cheating or not. It's a sure-fire way to make things unbearably awkward if word ever gets out or if/when the relationship comes to an end.