Generally speaking, all kinds of erotic materials, be they in the form of videos, stories, music, ect., are completely prohibited. Using erotic materials for educational purposes is also prohibited, because this is actually a weak excuse. There are always decent alternatives for these materials for those who seek what is decent.
As for the issue of the intimate relation between the husband and his wife, we should know that besides anal sex and sex during menses, which are absolutely prohibited, all other kinds of sexual gratification between spouses are allowed as long as there is no harm that may result as a consequence.