People are gradually learning to hear about, tolerate, and consider alternate sexuality and relationship forms, but it's still largely a world with a narrow definition of "good relationship" (monogamy being just one part of that definition), so sometimes people have inclinations/desires that don't "fit in," and wind up contorting themselves or their lives to "make it fit." Cheating would be one example; the secrecy is an alternative way to make the "superflous" relationship fit inside the mold. Essentially sweeping it under the rug to "make the house look clean."
Who knows how it would (hopefully someday will) look if society had a much more tolerant/open view/definition of (potentially) good/healthy relationships. I wouldn't even want to speculate; it's too complicated of a question. There would probably be a lot less secrecy, though. Not only less cheating, but less situations where people had to be "in the closet" about their lifestyle. It's too bad we don't live in that kind of world, but it's good that things seem to be slowly moving in that direction.
Love means never having to say, "Put down that meat cleaver!"