Despite the many middle-class perks -- the porcini risotto, the Tuscan bvacations/b, the predictably precocious children -- there is hardly any sex. At least not among Tsing Loh and her friends, mostly educated 40-something ... Cristina bNehring/b, in her recent book A Vindication of Love, also renounces the bounds of middle-class marriage and calls for dangerous, dark, undomesticated passion. She makes a good argument, battened with literary and historical examples. ...