When referring to adult male cross-dressing, "Sissy" culture refers to "forced cross-dressing", usually involving female domination in which a wife or female lover forces the reluctant male to wear women's clothing. This theme is popular to many men and women. For some transgenders living is stealth mode, they often feel like they are forced to wear men's clothing and often have fantasies of being given comparable levels of pressure to dress and look like beautiful and sexy women.
Given the female domination aspect, often the feminized male is also given a submissive or subordinate role, such as a little girl, maid, or "****".
The sissy fantasy is one that is not uncommon for women, especially women who have been intimidated, abused, or sexually assaulted by males at some point in their lives. For them, the idea of having the ability to humiliate and intimidate a man, making him do exactly what she wants, whether that is cleaning the house in the maid's dress, or giving her sexual pleasure or even just foot rubs, can be exiting and cathartic.
There are numerous books with "Sissy" themes, many with variations such as caging, bondage, or cuckholding.
Though it's a popular fantasy, even for transsexuals, it's not a reality that most people want to live on a full-time basis. A full-time lifestyle of that sort could easily become emotionally abusive, and would eventually wear on the self esteem of both the dominant and the submissive. It's similar to the exercise where one group of students were made guards, and the other group was made prisoners. With very little coaching they became so abusive that the experiment had to be stopped early because both the guards and the prisoners were becoming a danger to each other.
Like any power exchange fantasy, there should be safe-words, limits, signals, and a focus on mutual pleasure. If those words are foreign to you, you probably aren't a "sissy".
The word itself is loaded. For those of us who were transsexual or transgendered in elementary school, it was a word associated with violence that was just brutality. When a woman would ask me if I was a "sissy", that was the association I had with the word, and I would react with such fear and anger that it was obviously not a "fun game" for me. We might have had fun under different circumstances, but there were too many red flags for both of us at the time.