I think the society we live in gives side chicks way more props than it should.  I was actually thinking about this yesterday morning due to a local radio show that was interviewing some chick who was cool with being a side chick.  One of the hosts seemed to give the woman props simply based on the fact that this side piece made comments about "being comfortable with her choice" and "being more independent than most girlfriends/wives because she paid for her own things in spite of the man."  Also, the side piece made comments that she enjoyed the dynamic because its fun and exciting.
Well, of course being a side chick is fun and exciting....a side chick only has to deal with "dating" portion of the man.  He comes to her when he wants to have sex and/or go out on a date.  No bills have to be paid, no house maintained, no children to raise and no family to deal with.  Before this age we live in, a woman like this would've been blasted for such selfish ambitions and negligence.  But now she almost applauded....that's where the problem lies.
The whole side chick thing is not new and will continue because men want it and there are women who are fine playing the role.  However, I feel that whenever a side chick becomes an almost "ok" thing in society, that we are headed for some real family problems.  And when I say side chick, I delineate that from a mistress and definitely from a second wife(polygamy).  
The reality is, this is not just because men like having multiple women.  This has always been the case.  I would say that women have actually played a much larger role in the proliferation and exploitation of the side chick thing.  
Women are promoting themselves based on being the side chick of some famous/semi-famous dude 
Women are on IG posting pictures of themselves proudly proclaiming to be pregnant by some dude who is married/engaged/in a relationship
Women are selling their stories to gossip sites and media publications for money and their 1 minute of Mediafakeout fame
Women are openly proclaiming an affinity for being the side chick because its "easier" than a real relationship
Women are and have always dealt with men who they know are with another woman  
So, let's not do the "blame the men for female iniquity" game....its old and intellectually dishonest.
Peace