I've never heard any reference to this song being written about Sophia Loren, or indeed any specific individual - this kind of story does attach itself to any 'standard' becuse it adds to the attraction of the song. The most famous example is the possible subject of Carly Simon's 'You're So Vain', with the front runners there being Warren Beatty and Mick Jagger, but if either of them know, like Ms Simon, and indeed Mr Sarstedt, they ain't saying!