These are often about the rules we live by, often without realising them, or the effect on our life. Often they are felt or pictured as a trap that one cannot get out of, and sometimes there is a lot of fear involved in going beyond them.
They are like a boundary or an unknown territory that one cannot go beyond. Most of us interiorise our morals or beliefs into our dream life. In other words, we take as a truth that what is important outwardly is as important inwardly. In other words, you are as upset by a dream as if it had actually happened in waking life. Such mistakes make us feel things that are ridiculous. This happens with the morals we live with, by and may be necessary in waking life, we try to make them fit to our much bigger and freer dream/inner life and that causes conflict because the two worlds are completely different. So in dreams about sex we do not have to live by the same small moral world often necessary in waking life.