I don't think disengagement was ever designed to be a way to start. This comes after years of failure to be respected in your home.

I think the best role to start if you are not the custodial household is "fun aunt". Aunts will help care for kids, they will watch them, they are a part of their lives but they aren't the disciplinarians. They do the fun and cool stuff. 

If you are part of the custodial household, that isn't a realistic role. As long as your spouse backs you up, you can be the mom of that home. Disengagement comes when neither the kid or your spouse respects your role as a parent in the home. 

'disengagement' is the begining point, I think it's the fed up point. You don't usually start off disengaging because you have this dream of everyone singing songs around the campfire and big hugs goodbye w/ "I love you"s being said. Then reality hits and unfortunately we don't all get the same reality and for most of us it's a war zone in our own homes. So we try, we bend, contort, and damage ourselves mentally and then after all routes are closed down, we disengage. But the blame isn't ours, it's the parents. And the half of those parents we choose to spend our lives with bears responsiblity for their child. Disengaging is not, and should never be, us lying on the floor w/ 'Welcome' stamped on our heads allowing the skids to walk on us, wipe crap off their shoes onto us or damage, disregard, disrepect us. 

Disengage but do it with power. Set your bounderies and hold them. Make sure your other half knows them, upholds them, respects them. 

I think it all comes down to daddy guilt because of the divorce, BM feeling threatened and causing difficulty for her ex and demanding loyalty from her kids and those Disney notions that a stepmom and step kid are at odds so you can't really be trusted as a parent. Aunts uncles and grandparents can all demand respect but a stepparent won't get it. 

I think it's a self-fulfilling prophecy though. If you treat a step parent like they are jealous, with malintent and in competition with the step kids then that is what happens. It creates a tense and toxic situation and the best thing to do is to disengage. But one of the big reasons why men seek out a second wife to begin with is to have help with their kids so the relationship suffers because they want you to fill a role that you can't succeed at. 

Why is it? I mean the UNlucky teacher of a skid can place rules on them. The babysitter can. The camp counsellor or coach can. SM's are evil if they do. Often enough the guilty dad will tell the SM to cook clean entertain but NEVER correct the skid. Or worse daddio gives you his blessing to set rules on the skid. Once she does he throws the SM under the bus INFRONT of the shit skid. Happened to me once too much.  Damned if you do or  dont. I think thats when disengagement comes in. You just cannot win.

I have often asked myself.   I was always the "fun" one with friends and family children, I would get in the mix and play and engage with them, but they also respected when i would correct behavior.   Someone might get mad and pout in the moment, but it was soon forgotten and life moved on.   In stepfamilies, it seems those "corrections" and expectations are stored up as ways we have wronged our stepkids.  Then things get flipped to where the kids are in charge, and it seems to happen before anyone even realizes it. 

I have come to the conclusion that it starts out based in fear.   Kids are wired to want their parents together, and I think parents are wired to feel guilty if that is not the case, so they tend to overcompensate.  When the parent is single, it seems to work fine because the kids are happy with the time and attention, and there is no one else in the picture impacted by that.  When stepparent comes along, whether the SKid likes them or not, the SP is a threat to taking away time and attention, and things start to change.   The Skid doesn't want to accept the SP in an authority role, because they are fighting against losing the kid centric relationship they have with the bio parent, plus the dream of mom and dad getting back together.   The SP tends to disengage for fear the kids won't like them, which is a threat that there will pe problems with their spouse.  The bio parent is in the middle trying to keep everyone happy for fear of losing everyone, which is an impossible task.   They seem to tend to gravitate toward pleasing the Skid because of ther guilt, which leaves their spouse in the background feeling resentful and not an equal member in their partnership.  

Whether this is all true or not, I don't know, but as a SKid myself and now a stepmom, fear of loss or change is the common denominator in my case, at least at the root of things.  How that's dealt with seems to determine where things go, and if it's left up to the kids, they don't have the maturity or tools to properly deal with what they are feeling and there is not a solid ground for any family member to be on.  I wish there were more resources or even just awareness of what step families are actually going through instead of just the "wicked stepmom" generalities there seems to be.  I feel like in any marriage, if the couple is not working together at the top if the pyramid, it's going to crumble, but that doesn't seem to be how the world think stepfamilies should be structured.   

After being disengaged for more than 15 yrs, I stopped being disengaged in that I was no longer prepared to put up with bad behaviour anywhere near me.  The SDs are now in their 20s, and if they were rude to me in my own home I would have no hesitation in ordering them out of my house.  I think DH realises I have changed in this respect and I no longer collude with poor behaviour. DH's mother behaved very badly to me in 2018 and we have been estranged since then. 

Step parent's do not have to disengage.  

I think you see it suggested on this site because this tends to be a vent site where people (mostly women) are struggling with the steplife dynamic.  So it isn't a default, but it is an option and potential solution for women who want to try to stay in a relationship, in spite of the problems that come from having stepkids.

And..... honestly, in most cases, the real problems are not the actual kids or their behavior or actions.  The problem lies with the PARENTS.. and that includes the partner of the stepparents.  THEY are the ones who have almost always created these issues.. and perpetuate behaviors and responses that guaranty that no improvements will happen.  In concert with that, you have the potential for a third party in your relationship in the form of an EX/BM who may be high conflict and bound and determined to exact their revenge and bitterness towards their EX and the new wife via the shared children.

The kids?  they are feral as they have been raised.. they live up to the expectations of behavior being set for them.  Occasionally there is some true mental illness but generally when people come here and describe a 3 yo or 6 yo as a DEMON who is INTENTIONALLY destroying their relationship?  Pffffft.  The kid is behaving how your partner allows... it's lack of parenting and while kids may exhibit jealousy etc.. they don't generally have the same motive intent that is adult in nature.  So, while the kids may be a focal point of dysfunction.. it's often not the child's fault they are the way they are or put in loyalty bind situations which cause conflict.

And.. NOT every step situation is bound and destined for failure and disengagement.  I think that it's important to differentiate the fact that these kids DO have bio parents and that it is the bio parent responsibility to care and raise them.  That what the SP does is voluntary and is not an obligation to the child (though people might have an arrangement with their spouse to be a SAHP and part of that deal may mean caregiving for the non-bio skids).  So, while a SP does have an interest in what happens in their home.. deserves to feel comfortable and be respected by people in their home.. there may be aspects of raising those kids that they really don't need to be overly involved in.  Picking battles and deciding when an issue impacts them or not is something that many SPs have to do.  In the end, the bio parent is the one who should be the final say in parenting the child.. but certainly a SP should have a say in how the household operates.. and rules that may be applied to occupants of the home.

Honestly, if you have a horrid skid.. and a spouse that pushes you aside and has no interest in your input or advice?  Clearly that is not a good partner.  If they are expecting YOU to do the lion's share of parenting of their kids with no balance in responsibility in the home?  THEY are being selfish and not a good partner.  If you have a parent who allows a toxic ex to bankrupt your family via the court system.. you may or may not have a poor partner.. .but you may have a partner that is not able to protect your interests... unfortunately, in that case.. sometimes the choice has to be made.. are you better off WITH or WITHOUT that person.  A guy with 3 toxic BM's and never ending court battles and CPS? well.. it may not be all his fault.. but how much you decide to put up with is on you.. and perhaps some people are just "undatable" due to their personal circumstances.

I also don't think disengagement is an all or nothing game.  Certainly there is room to be friendly with the kids without taking on responsibility.  The age and relative expectations in your home will dictate what you can do there.  I know I was probably more disengaged from my OSD when she got into middle school... so from 9-12 we were pretty good. she became a person I didn't particularly care for at that age.. (very materialistic and kind of mean girlish).. and to this day.. I have a pleasant but not close relationship with her.. honestly, my DH doesn't communicate with her near as much as he does her younger sister who I have a GREAT relationship with.. with very rare instances where I may have disengaged .

As for society's expectations in stepparents? IDK.. all women should love babies and kids.. love homemaking.. and obviously it's not always as simple as just loving the kids.

When biodad wants SM to take 100% of the responsibility/blame along with 0% of the authority.

When biodad and the BM want SM to be the maid, nanny, nurse, cook, laundress, paralegal, administrative assistant, chauffeur but with no parental authority or say so.