THERE are 102 million men in the U.S. and 106.8 million women, and for a great many of them the most important thing in life is each other -- how they live together, or apart, and how they treat one another in the family, in the bedroom, on the job, in society. It is the oldest topic in the world and in some respects the newest.

The President and others rail against permissiveness, and yet the courts increasingly permit all varieties of sexual behavior and expression. The falling birth rate and the proliferating variety of mating arrangements may signify a new enlightenment, or the end of the old morality, depending upon one's point of view. Whether Eve was culprit or victim in the forced evacuation of Paradise is now an issue raised by some feminists. The female orgasm and male impotence are the serious, not to say solemn objects of growing scientific studies. Equality between men and women in all fields is an agitated cause.

All of which and more will be grist for TIME'S newest section, The Sexes. TIME'S system of organizing the news by subject has always been flexible. We have started new departments in response to trends or mounting concern (for example, Environment) and on occasion phased out others. The Behavior section, only four years old, has been covering many facets of the male-female relationship, but we decided that the field deserved its own section. It will appear in alternation with Behavior. We expect to treat the subject seriously, scientifically when possible, but humanly and at times, we hope, with humor. Says Senior Editor Leon Jaroff: "Obviously some new male-female patterns are evolving at different rates and in different guises round the world. It's a wide range of territory that will let us talk about the Pill one week and, who knows, love poetry the next." One sign of how topical the subject has become is the number of cover stories and other major articles that have already run in TIME. Kate Millett appeared on the cover in 1970 as a symbol of the feminist revolution. Last March we devoted an entire special issue to The American Woman. We have also had cover stories on the plight of the homosexual in America, on sex and the teenager, Sex Researchers Masters and Johnson and on the sex explosion in the arts.

The new section begins with three stories by Contributing Editor Virginia Adams, a veteran Behavior writer. One piece examines some new thinking about learning sex roles and describes the surprising case histories of children who have undergone sex change. Another is a report on British Biologist Alexander Comfort and his predictions for the future of unconventional sexual arrangements. The last article is a series of short communiques from the battle for equality between the sexes. One of these briefs makes clear that the contest is not always one-sided; a bank that had been providing free transportation to women employees found itself challenged by men workers.

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