extramarital affairs are relationship outside of marriage where an illicit romantic or sexual relationship or a romantic friendship or passionate attachment oocurs.
(check en.wikipedia.org)
婚姻以外的性关系,浪漫友情关系,或同情关系,非法律认可的浪漫性的,性关系的,浪漫性友情关系的(知己??),或同情关系。