To think he doesn’t need permission to go away?

No, YANBU. You have a healthy attitude towards relationships.
Your friend on the other hand, does not.

You're not missing anything. If your partner isn't competent enough to know or remember that he's not supposed to shag random people in Spain (assuming you're in a monogamous relationship) then he's not competent enough to BE in a relationship in the first place. But I think you would be better placed than your friend to know if that's the case; it's a weird thing for an outsider to assume.

She clearly doesn't trust her partner then

I’ve been married 20 years with teenage kids, and it’s never been a case of asking for “permission”. Running it past each other before booking to check there’s no logistical issues is courtesy and expected but would give short shrift to my DH refusing to “allow” me to do anything!
My mum used to make snide comments about how lucky I was that my DH “let” me go out so much. He’s not my dad or my jailer, I’m an adult with my own money and a fully competent co-parent so why would I need anyone to let me do anything? She unfortunately was only ever in relationships with controlling and abusive men though, so I think that was her experience.

We are married and living together with kids but did the same before that.
Never asking permission but a check that there is nothing else happening etc now it also includes checking the other person is ok with solo childcare and dealing with the kids for however long.

Your friends reaction just says so doesn't trust her partner or that is why she would be going away.

Not married, don't live together, no kids, no joint responsibility for anything, like even a pet or "someone needs to be in when the plumber comes", etc? Your friend is nuts. Obviously if there's childcare or whatever to be considered then overnight trips are to be discussed rather than announced, but otherwise...no.

She's being daft.
If you lived together or shared finances or had children then you would need to discuss plans because it affects the other but in this situation? No. He doesnt need your input.

Letting you know is polite- but why would he need permission?

If you have children that need caring for it's a bit different but otherwise I'm absolutely with you.

If I wanted to go somewhere, I wouldn't accept a man telling me I wasn't allowed because I'd only go and pick up a random for casual sex, let alone a man I didn't even live with.

DP stopped asking for permission once I said 'Only if you get a note from your Mum saying you can go'. I want to be informed in good time, not have it dropped on me a couple of hours beforehand and if it involves a significant expense, I would expect him to check that I agreed it was affordable first, but I am with a fully grown man, not a child.

My DP doesn't need to "ask" but I would expect it to be mentioned before it was booked, just to check dates etc and I'd do the same.

I would expect my H to check it was convenient and I would show him the same courtesy. That's not the same as "asking permission".
The same way we check in with each other to confirm which days we plan to WFH/in the office each week - try to stagger it so our youngest (age 12) usually has someone at home when she gets back from school and one of us can get started on dinner.

I'm married and I don't ask! We usually just discuss

That’s a ‘her’ problem.
Is she in a fragile relationship?

100% agree. Before we had kids we would have 1-2 separate holidays with friends most years (ahh the good old days). Never needed to ask permission.

Permission shouldn’t be required (checking whether dates clash etc is common courtesy). However, if the purpose of stag do is to go round strip clubs then I would hope my partner would decline the invitation

I’m married with dc with Dh and I don’t always check with him before I book to go away. 😂

He would generally check big stuff, but he would probably book something small that was easy to cancel without mentioning it to me. Though he’d also probably rather go for a weekend long prostate exam than a stag do in Ibiza. 🤣

There’s two things here.
First she thinks he should have asked.
Second she thinks you should have said no.

I think “asking” is reasonable - not in a “please can you grant permission” kind of way but in a “can I just check there’s no reason I can’t go” kind of way. Where really the only reason to say no would be some prior commitment they’d forgotten, or a financial reason if you have shared finances and would normally discuss that amount of money. And even then I’d say it would be a discussion not a “no you don’t have permission”.

Her being aghast that you haven’t forbidden him from going is weird.

We ‘ask’ each other if dates work because organising the kids is an effort with multiple schools and clubs (DDs special school transport is a job on its own atm), but we don’t ask permission to go away specifically.

The only person I know who actually asks their partner is someone who needs their partner to mind their children that aren’t his.

Permission no but checking with your partner before booking is common courtesy I think.

I think your friend is reflecting - does she shag about when away?

She seems to think that he’s off to shag about and that I need to put a stop to it.
Tell her that men who are going to shag about don't need to leave the country to do so.

"Permission" in this house is checking therefore no clashes, or its not going to cause a massive problem for someone.

The location / people etc aren't part of the equation. Its simple logistics.

There’s a difference between asking permission and checking that a trip doesn’t clash with anything that would need negotiation. One is controlling. The other is teamwork.

If there are young children, then I think either partner should ask if the other partner is ok with them going away.

But it would be crazy to ask permission when there are no children involved, even more so as you don't even live together.