''A couple is a conspiracy in search of a crime. Sex is often the closest they can get.'' All the present controversies about the family are really discussions about monogamy. About what keeps people together and why they should stay together. Now, in a book of 121 aphorisms, Adam Phillips asks why we all believe in monogamy, and why we find it so difficult to think about. Everyone knows that most people, however much they may love their partner, are capable of loving and desiring more than one person at a time. It may be reassuring, but it is in fact very demanding -- and often cruel -- to assume that only one person can give us what we want. At least in sexual matters, sharing seems to go deeply against the grain. Monogamy is so much taken for granted as the foundation of the family and of family values that, as with anything that seems essential, we are very wary of being critical of it. But, as Adam Phillips suggests, it is surely worth wondering why the faithful couple has such a hold on our imagination, and how it has come to be such an ideal.