I was in my mid 30's when I met my DH... Never had kids.. had never even had a strong drive to have kids.. Part of it was timing of relationships with wrong people.. but part of it was that I was not particularly maternal.. I never gravitated to babies etc.. personally now I am not comfortable being around especially small kids or ill behaved kids.

But, my SO had two daughters.. 5 and 9 when I met him.. he had a similar schedule. 

I would say that per your last blog.. some of the similar vibe was happening when he had his kids.. I think he was more on edge.. because "I" was more on edge.. he probably felt that I was not overjoyed with the house being overrun.. and when you are a childless person.. having two extra people in the home can be chaotic when you aren't used to it... it's not that they were themselves overly messy.. but when you take my mess.. and multiply by three that is a lot more clutter.. and dishes etc..

My SO was very good at trying to keep on top of it.. but obv kids aren't robots.. and there were times he likely sensed my irritation..which made him a bit more prickly.

I'm not saying my SO had a short fuse.. because it wasn't that.. but the energy of him trying to keep the kids from annoying me.. well.. that is a bit high tension anyway.

I mean.. part of it is your routine is messed up.. and t hen two more people to incorproate into your home.. and they aren't necessarily used to you.. your house.. boundaries or limits.. etc..  

Your SO is trying to balance his kids comfort and yours.. and it can be tense.

My best advice would be to make sure you have a way to decompress.. that you can encourage them to go out for a walk.. or you go for a walk.. do some quiet reading in a space..  don't allow kids in your bedroom.. you need a sanctuary where you can retire to.

You also can gently point out that hte kids are there to see HIM.. not you.. so you don't have to be present and participate in everything they do.

I never had a strong desire to have kids, but I thought I would under the "just right" circumstances, which never occurred. I've always loved kids -- other people's -- so I expected that I would have great relationships with DH's kids. Well, that fantasy went down the toilet. 
My ex doesn't date women with kids. He has a super low tolerance for drama and the kind of crap I've dealt with through the years.  
 

I never thought I would have a skid, but when I did I was completely naive to Disney Dad Syndrome, Mini Wife Syndrome, passive aggressive and conflict avoidant behavior.  I figured having a step would be like babysitting, you know it’s not your kid but they were taught to still be polite and respectful.  So, it was a long drawn out learning curve for me that not all parents actually parent their kids, not all kids are taught to be respectful and polite to adults, and not all exes are sane rational people.   

I was the opposite. Always wanted kids, loved kids, worked with kids, and i HAVE kids.

I just want to tell you that except in unicorn situations (maybe you meet them as babies or toddlers, the opposing parent isn't around much and isn't a big intrusion, and your spouse lets you parent, etc.) having skids is NOTHING like having your own.

You have these kids who were raised with different values from you. That's to say, when they are yours, you parent them to be how you want them to be. You can parent out of them the behaviors that you can't live with. As a stepparent, often any attempt to parent is met with defensiveness, by the kids, by their other parent, and even by your own spouse. 

As a parent, you are in control. You can get a CO and you are the one who has to agree to anything that happens outside the CO. You can ensure that the CO is followed. As a stepparent, you have to live with the choices of your mate and their ex.

As a stepparent, you are often being worked against behind the scenes by the other parent. They may reward the kids for talking bad about you or disobeying you. The "reward" is often subtle approval by the other parent, and this subtle approval can be very powerful. The ex also often exerts control over your spouse, especially in 50/50 situations or if your spouse is the noncustodial parent. Nobody wants their partner's ex influencing their home or their relationship. Nobody wants to see their partner's ex often or have the ex frequently calling their spouse or showing up at their house. 

None of these things even have to do with biology. They have to do with the fact of being a parent vs a step. An adoptive parent can do anything a bioparent can do once the kid is theirs. Some people say it's "biology" that creates the different dynamic, but i've known adoptive parents and kids who were adopted, and those situations are nothing like step situations.

Just saying, i hate to see people who were stepparents before being parents get "put off" having kids because they think it will be like stepparenting. It isn't. On the same note, if you don't want to be a parent and your step-situation is tolerable to you, maybe it can work for you. Example, if you aren't expected to do much extra work and if your partner has good boundaries with the ex. It could happen, but not to many people on this site, since the problems are what led them here. 

I've never had any drive for kids.  As it turned out, SS-31 is an only in our marriage.

I missed only the first 15 mos of the kid thing.

I like kids, they like me.  I am ambivalent at best about a BK of my "own".  My DW spoke periodically about an "ours" baby.  I have always told her that I have one and thank her for making me a dad.  He asked for a full meal deal adoption when he was 22 since I have always his dad.  So we got papers declaring officially what had always been the case.

...but that doesn't mean I hate kids, I just did not want to be a parent.  My partner has 3 kids, and the only thing that has bothered me is the fact I was led to believe everything was peachy, and that he and his XW coparent well, and the issues had died down as by the time I even started to date my partner, he had been divorced for four years.

  His kids are fine, they are good little people. The problem lies with the Opposite World parenting that is going on. My partner likes chores, discipline and his kids being active participants in society, and his X is just being the Disney Parent, riding on his guilt and never dealing with her own issues, and lets the kids do whatever with some performative parenting thrown in. 
  I do not regret being with my partner, and am not miffed he has kids. I'm annoyed by the attitude put forth that every parent is doing it right simply because they won a prize at the Egg and Sperm race, and everyone else in orbit has to kowtow to them unless they decide they want help, in which case you are supposed to pick up slack. 
  No, no, no.  Parents need to parent and spearhead the plans, and personally I'm happy to support them, but they need to get it rolling. My partner has made great strides in this department, but it's a constant back and forth as the XW just does whatever and anyone disagreeing is a big know-nothing jerk. 
  I never wanted kids of my own, but I'm fine playing a supporting role in the next generation growing up to be productive members of society. I can just do without the huffy puffy nonsense and lack of addressing lingering issues.

I never wanted kids, ever, even when I was a kid I would tell my family "I don't want kids" - like 10 yo.  LOL.

When I started dating after my first marriage ended my decision was to date only men with no kids or kids 5+. No teens. So I met my STBx. Things were great at first, but I quickly realized that I needed a parental role in the house but he wouldn't allow it so I disengaged. Not completely, but a lot. However even that little bit of engagement, coupled with a spouse who clearly didn't support me in a parental role made for a bad dynamic. I will never do it again, be with anyone with kids, let alone marry.