Sex and marital intimacy is fascinating, and W.L. is someone who is endlessly fascinated by it. And still trying to understand it better after all these years.
No claim here to understand everything about sex -- no one does. And his understanding is shaped very much by the person with whom he has been having sex for many years, though he has tried to find other perspectives to flesh out a fuller view.
The point that informs everything I will say is that the inner meaning of sex is spousal or marital. That is, the best way for me to direct my sexual desire is toward a person of the opposite sex to whom I have committed myself completely and until death. That is not just "one kind" of sex -- it is the way sex is supposed to be, what the sexual urge of human beings aims at, whether or not people know it.
This is not an argument about some "moral rule" about what sex should be -- it's an argument about the way the most fulfilling sex is. But, of course, we don't always see or feel what's really most fulfilling -- many people feel that they are flourishing when they are doing pretty self-destructive things.
For the most part, what you read here will be written from a male viewpoint, which is often different from a female viewpoint. But W.L. has to say that he has listened to a female viewpoint on sexuality and has learned a great deal from it. Which is really important, given that what might be called the "natural" female view of sexuality is superior, because it tends not to separate the sex from the person as much as the male view does. (But I do think it takes both to get the whole picture!)