I found out some ugly truths about the Protestant Duke, as the London mob called him in his heyday. It turns out that Monmouth murdered a night watchman in the course of a night of drunken partying, and Charles II issued a pardon rather than punish him for it. This may not have been his first murder.
Although he had an income of about 600,000 dollars in today's money, Monmouth was always in debt and was repeatedly bailed out by Charles II. He and his wife spent almost a third of their income on clothes every year.
He did indeed have mistresses. At the time of his death, he was involved with Lady Henrietta Wentworth, and seems to have had a child by her.
I also learned that James II and many others believed that Monmouth was not the son of Charles II, but the child of one of Lucy Walters's other lovers.