"Only the most willful blindness could obscure the fact that sexual intimacy is a “sensitive, key relationship of human existence, central to family life, community welfare, and the development of the personality.” The fact that individuals define themselves in a significant way through their intimate sexual relationships with others suggests, in a Nation as diverse as ours, that there may be many “right” ways of conducting those relationships, and that much of the richness of a relationship will come from the freedom an individual has to choose the form and nature of these intensely personal bonds."