Squeeze juice and pulp from oranges. Transfer pulp to a bowl; reserve juice. Mix marjoram leaves and garlic with the pulp.
Put the pork loin into a dish and season with salt and freshly ground black pepper, rub with pulp mixture. Cover and refrigerate for 2 hours.
Heat oven to 220°C.
Using the handle of a wooden spoon, borrow a hole lengthwise through middle of the pork, wide enough to push the filling into. Stuff hole with plums. Wrap pork with bacon so they overlap slightly; secure with toothpicks.
Transfer pork loin to a roasting pan with reserved orange juice and wine.
Roast, basting occasionally, until cooked through, about 1 hour.
Let rest for 10 minutes, remove toothpicks, and slice.