Anyone who has read Boccaccio, Chaucer, or Shakespeare knows that most themes have been used multiple times by multiple writers. That doesn't mean there are or can be new themes to be delved or powerful themes that have not been explored enough. The purpose of artistry in writing is to package these themes in new wrappers so the message and the ideas are fresh.
One of the most powerful themes and subthemes is sexual tension. This theme is easily observed in works like Romeo and Juliette. This theme is incredibly powerful and is exploited in most non-juvenile novels where men and women interact. The interaction of adult men and women almost always requires some degree of sexual tension. Sexual tension can be developed in three separate spheres of thought: natural, ethical, and moral. Moral use of sexual tension is a classical theme and revolves around licit and illicit sex defined by the boundary of legal, acceptable, customary, or promised marriage. Marriage is the general goal and the theme is propelled by the promise or hope of marriage. This is the classical theme in much of English literature especially in the Victorian Era, but is a theme and subtheme in much if not most of English literature. Examples are easy to come by--the Bronte sisters, Shakespeare, Jane Austin, and all. A variant of this theme is breach of marriage and or adultery. Examples here are well known, The Scarlet Letter is just one.
In the Twentieth Century and following, the sexual theme has morphed into one of ethical or natural sexual tension. These themes and subthemes are pervasive and generally intellectually crippled. Instead of marriage, the end of the theme is sex itself or a sex act. This theme is usually simply a subtheme, but focuses in sexual longing and desire driven by various romance based ideas culminating in the sex act with or without marriage or a promise of marriage. Ethical sexual tension, by definition, culminates with a stated or implied promise of some type. Natural sexual tension, by definition, simply ends in sexual congress. There is not a lot an artist can do with natural or ethical sexual tension--it certainly cannot really drive the theme of a novel although many have tried.
The moral sexual theme is one that is still well used in literature and should be--successful reproduction is the focus of human existence. Without it there will be no people to read all that great literature. The main point here is this theme is both critical and essential to literature and I recommend using the moral sexual theme or sub-theme to appropriately propel your writing.
Now, I will provide one of my real writing secrets. One theme that has not been used much is sexual tension in a successful or positive marriage. In fact, I know of only a few novels that successfully exploit this theme. You can see examples, amazingly, in some movies. Most of the time, in literature, movies, and theater the theme of marriage focuses around failed or broken marriage with an end of the change of spouse.
I am writing novels www.ldalford.com to exploit the sub-theme of sexual tension in successful marriages. Generally, the first portion of the tale is one of moral sexual tension with the result of marriage. Following marriage, usually authors ignore the concept of human sexual tension as though it didn't exist at all. As though sex or moral desire after marriage was nonexistent. In The Fox's Honor www.TheFoxsHonor.com, Sister of Light, Sister of Darkness, Twilight Lamb, and Regia Anglorum, I exploit the subtheme of moral sexual tension in successful marriages. I attempt to do this with class and without any salacious detail. This is a theme that is not new, but underused and I think the modern world needs to see this as a positive example in literature.