It's hard to say if they are 'popular' in general. There is a niche audience for transgender - related content, but when it comes to the 'adult' aspect, it may be less mainstream due to various sensitivities and regulations.
In many places, the distribution and consumption of explicit adult erotic audio stories can be illegal depending on the laws regarding pornography. It is important to abide by the law and stay away from potentially illegal content.
Adult stories that are just about normal life situations but targeted at an adult audience are legal as free audio. But when it comes to sexually explicit or inappropriate content, it's not legal. Laws around the world are designed to protect public morality and prevent the spread of offensive materials. So, for truly 'adult' in the sense of improper content, no, it's not legal to have as free audio.
Audio adult stories can serve multiple purposes. Some might be used for therapeutic reasons, like those that deal with self - help or mental health topics. They can provide comfort and inspiration to listeners, showing that they are not alone in their experiences. So, they are not solely for entertainment.
Most societies uphold values of decency and respect, and erotic adult audio stories conflict with these values. They are not suitable for public consumption and can have a negative impact on the psychological and moral development of individuals, especially the younger generation. Moreover, they may also violate laws regarding obscenity in many places.
There are apps like Storytel that have a large collection of audio stories. While not all are 'adult - only' in nature, they have a good selection that would appeal to adult sensibilities. They often have free trials which can give you access to a number of stories for free.
Society values decency and respect. Adult sex audio stories violate these values and can have a negative impact on individuals and the community as a whole. We should encourage the spread of positive and educational audio content.