That Mongkut, the future Rama IV, was a virtual prisoner in the monastery is quite an exaggeration. And the belief that Rama II “cut into” the throne is generally regarded as bunk, perpetuated by contemporary Westerners who did not understand the intricacies of choosing a successor in old Siam.
Chetsabodin, the future Rama III, was 16 years older than Mongkut, the future Rama IV, and had long played an active role in the government. But the former had been born to a concubine, the latter to a full queen.
As David K. Wyatt relates in his excellent Thailand: A Short History:
**Contemporary Westerners considered Rama III to be an illegitimate son son of Rama II, even a usurper. It was not that simple. For hundreds of years, Siamese had accorded higher status to princes born of queens, particularly queens of royal blood, than to princes born of concubines. But in law and in practiceall sons of a king had some claim to the throne, and it was up to the accession council that meton the death of a king to choose his successor.
What seems likely is Rama II knew he woud die soon and, convinced that the accession council would choose Chetsabodin [because of his already vast experience and expertise with governing], desired to protect Mongkut from political intrigues – and perhaps the internecine conflict that might follow the elevation of a concubine’s son to the throne. Had such a conflict occurred, Chetsabodin would have won with the support of the powerful Bunnag family [who remains a powerful force in Thailand even now in 2010].**
Other sources fully agree. (Bracketed comments above my own.)
By all accounts, Rama III himself was never entirely comfortable with his role as king, could never shake a nagging doubt that Mongkut, whom he remained close to, really should have been the one chosen and constantly insisted he would have been had it not been for his youth and inexperience. He performed many actions that signalled to his subjects that he considered Mongkut his heir-presumptive, such as inviting his brother to choose objects from the traditional palace of the heir-presumptive to furnish his new quarters in the temple, which he himself named Bowonniwet, the name being a play on the name of the palace traditionally occupied by the heir-presumptive (all of which may not sound like much to a Westerner but is chockful of symbolism to a Thai).
The young Mongkut would have made a terrible king. He used his time in the monastery – Wat Bowonniwet, which is still very much in existence and just behind the present-day backpacker enclave of Khao San Road – to good advantage. He was there at the behest of his father anyway, not his brother. I would call any Western attempts to portray him as a sort of Count of Monte Cristo are sadly misguided and ill informed.
Although he may not have entered the monastery on his own volition, he soon threw himself into the role, disliked what he saw in the various monasteries, and became quite a rofrmer, founding a new, more austere sect that continues to this day (and is still based in Wat Bowonniwet).
He was very active outside he monastery and would have had many an opportunity for a dalliance. Not saying he did, just that one should not apply traditional Western romantic notions to an extremely esoteric society for the sake of a good story.