I don't have a good tutorial at hand[*], but if you want to get started on not be limited by the options the online services offer then just install stable diffusion locally on your computer.
It's free, open source and easy enough to install - especially when using additional tools like the webui from Automatic1111:
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Just install that and some of the basic models and you're ready to dip your toes into the topic. Civitai is a popular source for that:
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It's pretty simple to get running and start doing some general prompts and messing around with it. Don't expect too much in terms of especially kinky things too, esp. actual sexual acts, since the models are typically not trained on that and struggle with anything beyond nudity. But for anything more, there are specialized Lora around. Think of them as some sort of add-on model that tells the AI what e.g. a blowjob should look like. Or specific poses, or cumshots. Or anything really where the base model isn't good enough.
Beyond that initial stage there is still a lot to learn though to refine and further specialize on what you want the AI to generate. Control nets, training your own Lora or finetuning, Hypernetworks, refining the base generation of one model with another...
I haven't really dipped my toes into that either, but am still quite happy with a bunch of pics I managed to get with just playing around with basic prompt / antiprompts.
From what I've seen, most of the paid online porn-AI generators are not doing anything different there either. Just a selection of SD models + some Lora. The only benefit is that you don't need your own computation power, but it comes at the cost of
a lot of flexibility and many of the advanced functionalities. And... well... money, because paid. And data, because you bet they keep logs.
[*] I'd be interested in good ones myself for the more advanced stuff like Lora and Control Nets etc. I haven't bothered looking into it yet though... I'm sure they are out there.