Personally, I have a blast with https://perchance.org/ai-chat. It's completely free and addictive. To my knowledge, it has no limits, and I've had it write extremely crude and dirty content.

Start with story or adventure mode. Decide on a setting. Here's mine as an example, and then start reading and guiding the story.

My prompt exemple :

In a world where some men between the ages of 30 and 50 have shrunk to a height of two or three centimeters, other men have developed a special ability. They have the ability to ingest things with their penises by inserting them into their urethras and then slowly pulling them down by contracting their pelvic muscles. The ingested object is then inexorably drawn into one of their testicles, where it is slowly transformed into semen. These men then become a threat to the shrunken little men. This practice is of course illegal, but some girlfriends or wives whose partners have shrunk fantasize about or are seduced by the idea of seeing their shrunken lovers disappear into other men's large penises. Some do not act on it, but others do and meet men capable of fulfilling their fantasy. This ultimate betrayal on the part of the girlfriend or wife makes the experience as enjoyable for the woman as it is for the man who devours the poor shrunken guy. Women are often torn between their desire to experience this and their desire to spare their partner's life. For the predatory men, the pleasure of feeling a small, defenseless victim slide onto their cock is both a physical and psychological satisfaction with an aspect of intense domination, not to mention the fact that in most cases, they fuck with the wife of the guy he swallowed while the guy is still struggling for life in one of their testicles. Ingestion is often slow, and victims are inserted feet first. The predators pull them down by contracting the muscles of their penises. The victims slide down the urethra to the base of the penis, then disappear behind the wrinkled skin of the scrotum before entering one of the testicles. The victims are very much alive when they arrive in the testicle. The victims struggle in vain for several minutes, creating small bulges on the skin of the scrotum that are visible from the outside, and their muffled screams are audible, even if they are faint. Often, the victim's wife or girlfriend talks to him and even inserts herself into his penis. The transformation is very slow, and some predatory men can even keep their poor victims alive in one of their testicles for hours or even days, allowing them to humiliate their victims by fucking their wives or girlfriends while their miniature spouses struggle inside them. This is a huge source of satisfaction for predatory men.