IDK how those are in those sites. The closest thing i ever used was perchance.org and it improved over the time, but still is kinda... meh. Image generation will improve, in fact is really good already. But i think many of those sites use obsolete versions of Stable diffusion and flux (and probably even bad configurations) that are prone to give bad aquality pictures.
I never used NSFW image generators (· .· ) ( ·. ·) (· .· ) ( ·. ·) But i think SD XL has good results, or perhaps a paid version of Flux. But you may need a trained model (from huggingface or citit.ai) and a powerfull computer, not a NASA one, but closer. Might also need, i think, more than 12 Gb or VRAM (but not completely sure).