They would have to admit that they aren't good parents.  They hate that their current partner can see through all the BS, therefore 2nd class citizen status.

Agreed. 
 

Unfortanately I ended with one of these types and I'm frustrated and sad. 
 

I think they're just experiencing their own tramas etc and it's best to step away and let them handle their stuff until (or IF) they're able to have healthier partnerships with other new partners

They still LOVE BM. It's BM who wanted the divorce. Not your SO.  They still want that "Happy Family". The one they dream about but never had.  They give BM everything and then think the new GF will pay for there mistakes.  Not only pay for them but pay for BM.  Giving BM so much CS they can't afford to live in the YMCA .  But GF will work and make up for everything 
 

Yup

Some guys will go down with the ship (dump off on SM to continue to please BM and the skids). Others will bail out (Put BM/skids in their place and protect SM). When things go south and there's no fixing it, the only way to find out which camp they fall into is to sink the ship (say Goodbye) and see what they do.

People, mostly men, will do whatever is easier on them.  It is easier on them to pacify the ex or the skids.  It's not really them putting the ex or skids first, they're putting themselves first.  They also won't stand up to their mothers.  They have to learn that it is far worse to anger their wives.  They need to understand that they are married to their wives, they have to live with their wives, and the wife will be there after the skids are adults.  The ex and the skids won't be the ones holding their hands and taking care of them in their old age.  

Unfortunately, it's not always easy or black and white in steplife.

I would say on here that the majority of stepparents (incl me).. are women.  And.. the majority of us have partners that are the NCP.. or at best 50/50.. For a variety of reasons.. but generally the court system still favors the mother as the nurturing and caregiver parent.. and see the (typically).. higher earning father as the financialy supporting parent.

So, you have men that are left in a tough position where they are already at a disadvantage.  They love their kids, but are given little time with their kids.. and just like the kids of many two working parent households.. they don't want to spend the small amount of time that they have with their kids fighting.. feel that they don't have the influence in the little amount of time they have.. and do not want their kids to start refusing to visit.  Since BM often holds the cards on visitation (even with a CO).. the BM can make it difficult for the NCP to have access to their child.  We see it all the time on her.. BM's not cooperating.. not enabling phone calls.. etc.. so the NCP wants to do nothing that will upset the apple cart with their kids or their EX.  

And.. the reality is that with the BM.. there is really not a whole lot someone's DH can do to "make her behave".  They can try for reasonable boundaries on communication, but in the end.. if the BM is a primary parent.. they may have to deal with them to get access to their kid.. and what are they supposed to do when BM makes a snide comment about their partner.. sure they could stick up for them.. but that's taking the bait and potentially escalating conflict that could impact their ability to see their kids.

And.. the kids themselves.. they are often in the middle.. they very quickly realize if they can walk all over a parent.. they also can resent being forced into a new dynamic that is fairly clearly driven by the new partner of their parent.  And.. we see this.. guys that didn't have many rules or expectations for their kids the couple days every other week.. they get a new GF or wife.. and all of a sudden.... there are expectations for chores.. for behavior.. etc.. that clearly aren't dad's idea.. so while they may be reasonable.. it's easy to see how kids could be resentful.. and if there is a bitter EX? they are happy to feed that fire.

And.. in the end.. I think that our SO's can become frustrated when we expect them to fix things that may not be fixable.. or may seem to have few real good options.  And.. there are cases where the step parents are particularly demanding and difficult too.. so there is def a mix.

Of course we want and expect our partners to have our backs.. but they also want to be there for their kids.. and many men just aren't all that practiced in raising kids...and may not even have them enough to make a huge impact.. so they end up doing the disney thing.. and resenting their partners for setting expectations they feel are impossible.. and yes.. when the kids are "attacked" it is also an attack on the parents by extension.. and a judgement and people often resent being judged.

I have found I got the most traction when I tried to craft things as being good for or helping the child.. vs complaining.. and fortunately.. my DH has a serious aversion to his EX.. so he communicated with her as little as possible.. but I know if she said stuff about me.. he would just ignore it.. and be off the phone as quickly as possible.

These guilty divorce parents. Decided to get remarried or get into a new relationship..   They should be going in at the same state as the first relationship.. SO is first. No otherthing /other person matters.  SO is on a pedestal,,,     But they go in still hooked up to the EX and there kids. ' Most likely the EX wanted out, '''They want back in with the EX. [ I know they all say that they will never go back... Don't care. ... But when the chips are down ]

OK..granted .. it's there kids. They have responsibilities to there kids.. they enjoy there kids. ..  Then STAY OUT of other peoples life's and enjoy fatherhood./ Motherhood...But once you involve someone else ...Things have to be different.  
It's  always amazes me.. That these people want to bring steplife  to a other person.. but they don't get involved with other people with children..They don't want to be SP.. 

'No  really wants to parent your kids.. No one enjoys your kids.  No one will be willing to take a bullet for your kids..

'No one wants BM/BF to control your home.  They want schedule,, what they have a say in . Not schedule that change 60 times a hour.  They want there money from there work going where they wanted it to go.  Not that CS is so high...New expenses ever day..  1/2 medical bills for ingrown nose hair..field trips.  Band... sports.. water polo.  That there money goes to run the home because parents are tap out .  
'you and the EX. Went on your honeymoon alone. You went on vacation alone.

Now you want to bring the kids on your honeymoon because BM needs surgery on a arm boil and will be sideline for the length of said honeymoon/.  And next summer you want to go to Lego land for vacation.