In a small bowl, whisk together lemon juice, honey, mustard, salt and pepper until blended. Add oil in a slow, steady stream, whisking constantly, until dressing is thick and emulsified. You should have about 1/4 cup.
Combine cantaloupe, pineapple, blueberries and nectarines in a large bowl. Chill until ready to serve. Just before serving, toss fruit with dressing and stir gently to coat. Serve cold.