Interestingly enough, the 3rd son married the mother of their illegitimate child. He was only 19 when she got pregnant. In an effort to protect their privacy, he has renounced his claim to the throne, and that of his children.
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His uncle did the same thing 20 years ago when his first child was born out of wedlock.
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It is possible that an heir who wants to marry a Protestant would also renounce his claim to the throne.
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I think that the only reasons the rules haven't changed in the UK is that nothing has forced the situation. If William wants to marry a Catholic, I think parliament will change the rules. Also, if William has a daughter as his first born I think they will change the rules and put her higher than a younger brother. A daughter has only been born first a total of 2 times in the last 350 years.
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If William gets someone pregnant (presumably Kate) then I don't know what will happen. According to the current law, you don't have to be married when you get pregnant, but you must be married before the baby is born. A marriage "after the fact" still makes the child ineligible. In other countries the rule is different. In Monaco if you marry afterwards, the child becomes eligible. If the Prince of Monaco would theoretically marry one of the two different mothers of his illegitimate children, they would be next in line for the throne. Subsequent divorce would not change the situation. However, people would be outraged so he is unlikely to do any such thing.