I think cheating is accepted more because it's easier than swinging or being openly non-monogamous. To be open requires the fearlessness to be honest about what you want and a willingness to go through the myriad of issues like jealousy and ego bruisings that come along with opening up.
Cheating requires that you just sneak off and fuck somebody.
Some where this topic took on a new life around honesty and judgement and a few other things.
People are people...very true. Cheating is in the news everyday. Just pick up the local copy of People on the way out of the store and the subject is on the cover of every issue. Who's cheated on who this week. It sells really well obviously.
The more something becomes a common thread of discussion it's loses it's shock value. Cheating has lost it's shock value years ago...swinging though I'm sure will be one of the next big ones as it rises in popularity.