Heather Carroll remembers Maria, Duchess of Gloucester, a niece of Horace Walpole. To quote:
After three years of marriage Maria was a widow with three young children. In those three years she had also made a name for herself as a delightful society lady. Maria had become close friends with Lady Coventry and Horace Walpole delighted himself to report that the beauties would be mobbed by onlookers when they took walks in the park. However, she was still not thoroughly protected financially. She needed another husband, and there was no shortage of eager candidates. She had already rejected the Duke of Portland (a duke!) when Prince William, Duke of Gloucester became acquainted with her. A romance blossomed, but it was a forbidden romance for Maria's illegitimacy prevented William from being able to marry Maria.
His brother, King George would need him to form a proper attachment. Their chances of marriage approval diminished completely when the Duke of Cumberland enraged their brother by marrying a commoner. King George saw the marriage as a betrayal and made William promise not to do the same with Maria. William gulped, and agreed in an attempt to calm his brother's anger. When Maria became pregnant though, in 1772 he had to finally admit that they has been secretly married in 1766.