Quoted from iluvbertie: "If Harry were to marry before William I would expect William to get a Dukedom first anyway to have the older title."
I don't think Harry will be "allowed" to marry before William. It's a good way to control the distribution of titles. All the Queen's sons married in birth order. A coincidence --- perhaps.
Not really a coincidence at all. When Charles got married, Andrew was only 21 years of age and Edward was still a minor (17 years old).