That he got married is of no import here. Until he and Anne-Marie had their first chlid, the presumed Heir to the Throne was Princess Yriny.
Upon the death of King Paul, in March 1964, Diadoch Constantine became King and Princess Yriny presumed Crown Princess. King Constantine married in Septmeber of that year and he and Anne-Marie had their first child during late 1965, that was Alexia. Upon her birth, Alexia became the presumed Diadoch until the birth of her first brother, that is, Paul.
Let me give you another example to make it better understood. If, say, King Constantine, after his wedding to Anne-Marie, lost his life, before he had the chance to father children, his youngest sister, Princess Yriny would have become sovereign queen of Greece and Anne-Marie the queen dowager. If Constantine became unavailable while Anne-Marie was pregnant, then upon its birth, the child, whether boy or girl, would have become king or queen (but with regency until his/her coming of age).
By the way, the succession system in Greece, then, was cognatic primogeniture (that is, male preference over eldrer sisters but allowing females to become sovereigns in the unavailability of brothers or offspings thereof) as is still the case in Denmark, Great Britain and Spain.
