The "horizon" of sexuality is the time framework within which you look at a sexual act. There are many different horizons of sexual union, and the act of sexual union may look somewhat different from within these various perspectives.
It can be viewed as part of an evening or a morning. Or it can be viewed as part of a particular week, or month, or year. with all of their unique characteristics.
It can be when this has been a tough day, or on a day of many delights. It can be during a challenging year or a relatively uneventful one.
It can be during a time when you are making love often, or it can be the first time in what feels like quite a while.
Besides time in the narrow sense, it can also be the time period of a relationship. So it can be set in the context of a marriage: on the honeymoon, or during the childbearing years, or after menopause. It can be a particular period of a person's life: youth, young adulthood, middle age, or old age.
What is popularly known as "make-up sex" -- having marital sex after you've had an argument or spat -- is another example of the framework of sexual union affecting the kind of sexual union it is.
It can even be set in a context that is broader: especially the family -- the sexual union that is part of a broader story that goes beyond one generation of the family.
Each act is a part of an overarching narrative, shorter or longer. Just as a paragraph looks different when viewed as part of a page, or a chapter, or an entire book. . .
This is one reason why sexual union between two 70-year old spouses will look very different to them than to an onlooker. The 70-year old person with whom you are uniting -- with its 70-year old body -- has a history of many years, with you. The sexual union is deeply influenced by the memory of that body over the course of their marriage. So even as the body declines and loses its natural beauty, the spouses see much more. Memory can play a powerful role.