Selfish or not? Didn't help his ex out

You were absolutely right to draw your boundaries and make it clear that you are not the fall-back guy for anytime the actual parents of the kid fail to parent. There is no way you should feel guilty, it is not your responsibility.
It is bad enough to be the "reliable" backstop when it is your own kids, so the other parent gets to take you for granted!
I hope you had a good gym session. Stick to your guns.
If it were me, I would tell them that anytime they need me to step in to look after SS, they need to give me enough advance notice to rearrange my own plans. CFs!

There’s going to be times when a child’s two parents may have difficulties with certain events/etc and the step-parent will need to step in.

Nope. The 2 parents can sort it between themselves, like other divorced/separated parents do when there aren't new partners in the mix.

Why?

Because step-mum said no, she has existing plans. And it's not her responsibility to sort if if she doesn't want to.

And again, don't see the step-dad getting a hard time for saying no on here...

Something as important to the child as a presentation the parents should have made time for. It’s not up to you to pick up the slack. The kid probably wouldn’t have wanted you there anyway, he’d have wanted his Mum and/or Dad.

Yanbu, stepparents being held to a higher standard than the actual parents, as usual.

Op obv isn't coming back to answer some of the questions I said and others

Mainly being

Why didn't the dad know if it was important to his son

The dad should have contacted the football coach to get all important dates /get emails /texts

The mum could have texted the dad to say wow henry has a presentation can you make it in 2w time

The son at 9 should have told both parents if important to him

How far away was it - 30m in taxi would have been a cost both way but if was 5/10mins why didn't they do that

Why didn't the mother book the afternoon off or let the dad know in advance?

Why didn't the dad leave work or put his name down at school/clubs so he knows what's going on?

Why didn't the step dad change his plans?

Why didn't the grandparents take a taxi and mum/dad/both pay for it?

Why is it all on the OP?

6 adults involved with this child, but somehow OP is the wicked step mother?

I presume because this was happening during her step-son's time with his mum. If it was during his time with his dad (3 days per week), I assume that OP's husband wouldn't be working or would have made some other arrangements for child care.

There is no legal relationship. She is a stepmother because she married her step-son's father but she has no legal rights or responsibilities for her step-son.

Any care that she wants to give to her SS outside her DH's responsibilities as a father is entirely at her own discretion.

Absolutely not your problem.
Stop being obliging.
It is a thankless job.
She didn't bother telling his father to keep him away from it.
It bit her on the arse this time.
OP, keep your distance and keep her blocked.
Don't be guilted by your husband.
This is not your child.

By her calling you names, you have solid proof how little your help is appreciated.
Stop doing ANYTHING for her and cite those texts as the reason.
Keep her blocked too.

Oh get over yourself please.

The child has a mum and a dad and grandparents.
Any of them could have prioritised their time for this poor kid.

Wild how that’s supposed to tug on OP’s heartstrings more than those of his actual parents.

OP had plans. Said plans may be less important to mumsnetters but they were important to OP and she was right to prioritize that. She organized her own childcare in advance so she could have that ‘me time’, and it’s not on her to accept someone else’s failure to organize themselves as her responsibility.

Yes, it sucks for the kid, but that isn’t on her. She’s not there to pick up the parenting slack for her husband and his ex.

Yes i agree. And also with the other posters pointing out the same. My dd and her ex co-parent very successfully with a shared calendar. There are occasional glitches but in the main they both know about things well in advance. They both want what is best for their child.

Maybe the actual parents should have thought of that earlier? Seems pretty standard.

I feel sorry for the kid, I personally would have taken him and rescheduled my gym appointment or skipped one this week as a one off.

Maybe you/ ex/ DH need to get some sort of calendar app that his mum can add stuff too so you are all aware in advance and can sort out who is going to which event. Saves the son missing out on future.

I'd have taken him op Shame you've prioritised a gym session over your step son.
I have a football mad child these events are so important to them. Yes it's unfair you were asked as a last resort due to circumstances and his actual parents should have prevented that but you had an opportunity to make a difference for that child and didn't and that's dreadfully sad.

The thing what annoys me most is that if OP was doing the same thing as what the stepdad is doing (spending time with his children), she would have got an absolute pasting on here for it.

Why didn't the "poor lad's" father have this in his diary? Why didn't the grandparents use a taxi? Why didn't ex-wife's husband change his plans?

Why was this on the OP???

Why didn't her partner cancel his plans?

Should have put this in the op!

Why is the man exempt from scrutiny?

he isnt her kid how dare she do this or that, but I can't take him to school tomorrow morning can she?" over the years.

this is spot on from OP and likely many many women with step kids, you've to be a skivvy, have no say and never speak up.
There were 4 other adults that could have changed their plans , this is not on OP.

If it was so important, Mum should have taken him!

OP you sound like you're between a rock and a hard place. "Should have" cancelled one of your measley 3 hours of pre-planned and costly self-care a week because SS should have been prioritised by one of his parental figures and yours was the commitment that could have been cancelled with least severe consequences (and handily none at all to exW).

At the same time this is a pattern for exW not to inform DH of things in advance, and of expecting OP to pick up shit she has dropped last minute, while reserving the right to slag her off when she does other nice things for the kid. I too would put up boundaries so that exW couldn't continue to treat me like her lackey while having such an obvious problem with me and disdain for me. I agree that if you cancel PT for this one time that instantly becomes the expected norm in exW's (and possibly DH's) mind, and today's kindness will be abused for future purposes.

Meanwhile the poor kid didn't get to attend his presentation due to his mum's behaviour and OP needing to stand up to that. I do feel for him and hope that him missing out doesn't become the norm, for his sake. That sort of thing can be damaging.

Jesus, step-parenting here on MN appears not to have moved on from Cinderella. Why do step-children get such a hard time?

It's more like if the step mother (and only the step mother, step father gets a pass) doesn't drop everything for the children they are immediately the wicked witch of the west. But they mustn't actually parent the child, they are only the step mother.

I'm amazed at how many people don't seem to know the difference between a gym class and a PT session!

Well, I think this is selfishness to an nth degree. The poor lad's parents were unable to take him - and clearly, this wasn't just a match/practice as there was a presentation, so was important to him.
But you were not prepared to miss one of your thrice-weekly gym sessions to help out? Because you have a job and a toddler and you need 'me' time?

Jesus, step-parenting here on MN appears not to have moved on from Cinderella. Why do step-children get such a hard time?

I'm surprised that neither of the parents were able to get in touch with another child's parents who were going to the presentation, and get him a lift with them.

I also wonder how much a taxi for the ex wife's parents would have cost. Perhaps not more than the c.£40 that OP would have been out of pocket by if she'd have ditched her PT session. I presume one of them could have picked up DSS and GP after the presentation.

ExW can't have it both ways. Either the presentation was big and important enough that DH should have been informed and invited with notice, or it wasn't a big deal enough to inform and invite DH so she can't then get mad at him or OP for not being available last minute. Perhaps ExW needs to be more assertive with work and have told them that she couldn't stay late due to prior commitments.

Great post.