What is Clayton's relationship to Marie St. Allermain in the novel, Whitney, My Love?

Marie St. Allermain is Clayton Westmoreland's ex-mistress. Marie St. Allermain is a singer who performs all over the world. Shortly after Clayton and Whitney are married and suffer an estrangement, Marie St. Allermain performs at a party they are attending. Marie St. Allermain senses the tension between the couple and suggests to Clayton that she would be open to becoming his mistress once more. However, Clayton knows that this would mean social suicide for Whitney, so he refuses. Clayton is not in love with Marie St. Allermain and does not want to be with her for emotional reasons. However, Clayton enjoyed his time with Marie St. Allermain and thinks about an affair for that reason but ultimately refuses out of sentimentality for his wife.