To me, a failed parent doesn't have to be a crappy partner. Like, if someone admits that, maybe they had kids too young and mistakes were made. If they own that and do not let the kids or the ex ruin the relationship, they are still possible partner material.

To me, a lot of the partners talked about on this board are just sh!tty partners on top of being failed parents. There is a core personality flaw, usually selfishness or narcissism, that creates failures in all their relationships. There is a lot of cowardice, in failing to set boundaries with exes or kids or even their own families such as toxic MILs. Maybe they themselves thrive on drama and create it, consciously or unconsciously. There's a reason a lot of them were single and available.

It's possible for a person to learn from their mistakes and do better in the future. But - at some point, you have to cut bait when you've repeatedly stated your needs and they still aren't met. A lot of people become too emotionally attached, keep fooling themselves that their partner can change, or are afraid to start over. A lot of women become financially dependent on men, too, and when prince charming turns out to be a poisonous frog, they don't know how they will live on their own. 

I personally put it down to ignorance. I was in my 20s and looking for someone who was mature and stable (plus my husband is generally a pretty nice guy). I truly could not fathom the depths of step hell that awaited me however. Crazy BMs, unparented children, $ flying out the door... etc etc. It just never occurred to me that there would be all these problems. Looking back, had Little Idiot been younger when I met DH, our marriage would not have survived. Being a stepparent has taught me that I'm really not cut out to raise children. Thankfully, DH doesn't seem to interested in being a terribly involved father either (he doesn't even text her except to say "Happy Birthday" and maybe ask if she's coming for Christmas) so it's worked out for us. He is really enjoying his low-stress, child-free life with our pets these days. 

My ex, although a nice guy, was not a good parent to his daughter.

He totally enabled her and this was the major contributor to her failure to launch. Good parenting means equipping your children with the tools to be independent and make a go of it in life. I used to try and explain this to him but his stock response was

"I don't want any hassle. I just go to work, come home, mind my own business, then go back to work the next day"

Total cop out.  Too scared to confront the situation and do the hard yards to make his daughter independent.

A lot of first marriages can fail.. due to immaturity.. due to rose colored glasses.. and due to the fact that a LOT of people are just not very nice people.

2nd marriages?  well.. the chance that you end up hooking up with someone who is not a very good or nice person.. goes up.. and the possibility that YOU are not nice or good does too.  Then there is the potential baggage and history that has to be dealt with.  And.. kids?.. those are whole human beings with their own issues and impacts right? 

I mean.. I see situations on this site where the SP has married someone that is not only a poor parent.. but a poor partner.. that treats their partner terribly.. and the children fall in line there.. often a HC EX is involved.

I also see situations where the SP is just not a nice person either.. maybe justifiably or not in reaction to a child that was not parented well.. or a HC EX.. or just had some fairy tale that they would ride off into the sunset and have their "new" little family and don't like the Ick of the old kids or EX having any impact.

As far as many of the "poor parent" situations here.. a lot of time it's the father we are talking about.. and gender roles do feed into that.. most often the mothers are the one doing the primary raising of the kids.. and the dad's just don't have much of a clue.. and then there is the fact that father's custody is often less.. which means they are the ones that are afraid if they aren't fun enough.. easy going enough.. that the kids won't come.  If they don't bend for BM.. she will withhold the kids. etc.. Kids figure out the paths of least resistance.. and when a new SM comes into the picture.. tries to instill all these new rules and  boundaries? is it any wonder the kids resent it.. even if dad IS on board. with the changes.. it doesn't take a rocket surgeon to see who the one is that brought these new and TBH.. unfun changes to the house.  (like responsiblilty and chores are good for a kid's development.. but fun..uh no).

Then you throw in human nature.. a woman not liking to see another woman "mothering" their children.. it's not a good situational place for kids to be raised honestly.

And.. there are a good number of people on this site.. (or their spouses).. that probably should not have been in relationships based on their situations.. the red flags abound.

People who have children, For what ever reason. Then feel divorcing is a good way out. Instead of staying and trying to. Give the kids a normal life.  Then these people divorced find another partner and think that partner wants a happy family with SK,  The BF. One they had kids with, is doing nothing, but new SP should do all.

Therr is just something wrong with this thinking. In my case my DW was a SM for five years and did nothing with SK.  They did not sleep over, did not ever go anywhere I.e amusement park, fair. That was OK in her eyes. Had reason. Appartment to small.  You have four SK but get one bed room apartment ?  
But with me,   I havd SK 24/7/365. That's was OK. BF did nothing. But I didn't do enough? 
In all the years and haveing a normal discussion, she could never see the difference, That if it was OK for her Why wasn't it OK for me.

i did for my SK, don't get me wrong. Amusement parks, Disney World, Vacations  at home 24/7/365.  But I always didn't do enough.  There has  be something wrong with that thinking.  Just makes me wonder if they are out of it in first marriage too

I think a lot of single parents are back on the market FOR GOOD REASON, and people seeking partners should proceed with extreme caution when vetting one. Instead of portraying them as noble people struggling to raise their kids, maybe we should slap a "Huge baggage, buyer beware" on them.

But that's speaks to another problem - the overall dearth/blackout of information about the pitfalls associated with step life. The Single Parent Complex needs fresh meat with financial resouces and labor to keep it afloat, so it's up to us SPs to get the truth out.

Unless you live it,  You really can't understand it.  Plus there bate and switch.  When dating everything is really good. Once you get into the relationship, your status in the family goes down fast.  From the numbe one person to the servant.

That your life gets controlled by " for the kids". The " kids didn't asked to be born". Kids " didn't ask for the divorce "  

That why most SP will never do it again, Never get themselves in a disfuctional relationship again 

DW married her ex. He was such a loser,  could never understand how she could not see it. He was a man who had four kids, divorced his wife and move 2000 miles away. For no good reason except he wanted to move.  Never seeing his kids.  Didn't want to pay CS. Was made to.  CS or jail.  They didn't have the SK at there wedding.

Returned rented a one bedroom apartment, So no SK visitation, no place for them to sleep.  Never took them anywhere, place.  As an amusement park.  Never payed CS for the kids he had with DW, never took them, was never could stop drinking, needed a drink at 9 am.  DW would call police for a DWI check.  But in Sparent land I was the bad guy.  I didn't do enough 

I never had a real problem with my SK. The problem was with my DW.  I had them 24/7/365. No one would take them.  Just needed adult time and that was a problem.  That was asking too much.  It cost more for the babysitter then going to dinner someplace.  Yes I had to pay for a babysitter if I wanted to go out