I'm not sure if an API is that common an option with AI hentai generators in the current market... but there are already many blogs recommending online AI tools and I think you could check them out:
https://ambcrypto.com/blog/top-5-too...-free-in-2023/
https://mspoweruser.com/7-ai-art-gen...w-nsfw-images/
Note that some of the tools are light-weight tools that generate images using only text prompts like SoulGen, picso, novelAI, while others may require a bit more technical skills to adjust the model parameters like in Stable Diffusion.
These tools each have their own style and if you play around a while, you'd soon get the hang of their prompt engineering skills and you can generate desired hentai images more efficiently.
Hope this helps!