I think it happened the same with Simba and Nala, they fall in love as adults.
Zazu with 'It's a tradition going back generations' I understood he will mean to best friends in its case, that as adults fall in love; then Simba said that it will be gross because he sees her only as a friend, and Nala agreed with him, but they didn't knew that as adults they will fall in love as finally happened.