If you write down your porn ideas:
How do you do this?

I have a text file on a USB stick.
Used to have a small notebook, but a file is simply more convenient.

The ideas will probably stay on the stick.
Meaning I won't write a story or pay for a commission.

Maybe there are better programs than text editors?

pen and paper, locked in my room

I usually just get really drunk and tell anybody who's willing to listen (and probably some that aren't) while I get drunker and drunker and forget what I even came up with. But people remind me. They don't stop reminding me.

I use Goog Docs. Has enough features for me and good organization tools

I tend to "sketch" my ideas down into Notepad++ and promptly forget about them. When I find them again later, if I still like the idea I migrate them to a google doc and try to flesh them out more.

Notepad .txt files.

For BloodRayne PSO crossover 3, sex happens in the main story a few times. Never planned it to happen, just did. One of the characters transforms into a catgirl, then... yeah.

For Gedol Odyssey series, another character transforms into a catgirl, and well... Sexy times are not in the main stories this time, but separate ones along with other random stuff.

random 034: Torri x Terri
random 035: Are We Best Friends, Or Are We Waifus? (Popi x Dragonetta)
random 036: Mira x Lina
random 037: Lilith x a Coffee Shop?!
random 038: Popi x Dragonetta x Hilda

Notepad++ for me. Then if I'm still interested I edit it down and run it as an AI dungeon prompt.

google docs for me

That depends on me, but I'm more of a plain ol' pen and paper kind of guy. The alterative would be Notes apps on mobile and Notepad o Windows

There surely are some people who also like to do it secretly.
Paper and pen users have many times the feel to it argument and the wherever you want as long as you have pen and paper at hand but also anybody could stumble over it. Also you have pen and paper maybe an eraser. So the features you have are write, draw and erase. As long as one of those is not available anymore.
If you move to software to edit you can add text or images, copy paste, move around and share dependent on your software.
Having it on cloud gives you the advantage of being able to do it on any device as long as it has access and the software to edit it but privacy and internet are always a matter for themselves.
An USB stick is small and can also be sometimes just put on your key chain. Encrypting those is possible, too.

I do stuff on my encrypted external hard drive but only because I'm doing some game development and have some bigger files than texts.

I'm no writer though. So I don't really have some inside knowledge about which software offers which features and which features are really good.