Surely Mary Stuart and William of Orange were married long before James II was deposed.

Yep. 1677 (We got a day of school for it!). William and Mary "ascended the throne" together in 1689 to rule jointly, as King William III and Queen Mary II. UK had had Queens before, so why the insistence on a King? And why jointly? They were first cousins, both having Charles I as grandfather. Apparently parliament wanted Mary as Queen, Mary wanted her husband as King, and William wanted the throne by inheritance. Crowning them both seemed an acceptable compromise.

Rod