When he thinks he's talking to Gwen and telling her he loves her and he's actually saying it to Mary Jane. The way that scene is set up gives the impression that in his mind, Gwen is the one he really loves (We're shown him saying "I love you" to Gwen and NOT MARY JANE), which is silly. It's not a competition and hasn't been one for over 36 years.
When Gwen was alive, she definitely was the one Peter wanted to be with. While they weren't shown as officially being engaged, Peter did, at least once, mention them getting married as though it were mutually understood that it would happen. But you can't marry a dead woman, at least not under the comics code, so he went on with his life and found someone else that he loved. How are you supposed to quantify both loves and compare them?