My feeling is this: If they are HAPPY, and they aren't hurting anyone, then who's to say it's "not right"? And furthermore, who DOES know what's "right and wrong"?
So many people in this world are miserable. If being with an additional person (or, additional people!) makes them live a better, happier life, then I say that's wonderful!!
Personally, it's not for me..and that's only because I currently have one man only that I love, and am attracted to..But, who's to say I wouldn't feel differently if I was not with my husband??
Also, one could say that people with more than one lover are getting the 'best of both worlds' (to use a cliche!)..Especially those with male AND female lovers...Those people will never be missing anything relationship-wise!