The British Sovereign is also the head of the Church of England. By tradition, a prince in the direct line of succession was supposed to marry a virgin, which is the Christian ideal but that typically was only expected of women. Also, she was to be of a noble family, so that she would have been raised with the complex etiquette and diplomatic standards, and some experience with managing many servants without identifying with them, since the royal households are like museums, are used to host other heads of state, and take a lot of staff to maintain. Also, the virginity would avoid the scandal of some other man telling some tabloid how the Queen is in the sack, or her making a comparison to someone else's sexual prowess, etc.
So, between Camila having ditched Charles for another man and the royal tradition staring him in the face as he became well into his 30s, the family looked around for someone "suitable" based on those parameters. The wealthy in England, and historically in most advanced cultures, would often arrange marriages to be sure that property was not lost, or to form political alliances with other nations. (Although Elizabeth appears to have married for love, notwithstanding that Philip was from a noble family.) So Elizabeth and Philip encouraged Charles to marry someone like Diana.
Before the rise of birth control, the restrictions on women were much more severe than on men, and especially on royal wives, to be sure that any children would be of the same bloodline tied to the succession to the throne.
The Crown has modernized to the point that Charles had fooled around with plenty of women before marriage without the Church insisting that he marry anyone with whom he had sex, which would have been the case historically, to avoid the problem of bastard sons forming armies when they grew up to unseat the legitimate heir to the throne. Charles' civil divorce seemed preferable compared to King Henry VIII's solution 500 years ago, of imprisoning or executing a succession of wives who failed to produce a male heir, and arguing with the Pope for annulments from the rest, eventually breaking away from the Holy Roman Empire to found the protestant Church of England. Much bloodshed ensued before that schism was settled. These days, the Church of England has given so much ground to pop culture and the so-called "Sexual Revolution" that it is nearly impossible any more to declare a moral scandal.
For most of his life, Charles, while not obviously mentally ill, has been a few bricks shy of a load when it comes to acknowledging the conditions of his countrymen of lesser rank or privilege. He doesn't act like a total snob in public, but is apparently spoiled rotten and makes petty demands on staff to cater to him in private. He reportedly has his own golden toilet encased in a luggage-like trunk that must be taken with him on any excursions or trips, and there is a servant whose job title, Groom of the Stool, describes his function: to dispose of the contents. Charles is fussy about his food, his clothing, his gardens, his stable, his furnishings, etc. etc.
For a long while, the public has been having to educate this prince about what they will and will not easily tolerate. In the absence of any true repentance for the mess he made of his marriage to Diana and the scandal that his affair caused to ordinary people who can be health-broken or financially ruined by divorce, the Brit public did not want to accept Camilla as Queen. As other posts above mentioned, only her long years of behaving discreetly since they married in 2005 has made her look good by comparison to Sparkles Markle. Also, Charles had a long fascination with Islam and made comments about not wanting to assume the traditional title, Defender of the Faith (Anglicanism), but rather make the title an ecumenical mish-mash, "Defender of Faiths." The ethnically British public has not been at all pleased with that idea, either. In the past decade, Charles has been making more public references to Christianity. Since he's in his late 70s, it's about time.
When did the Church insist that a King or nobleman must marry anyone with whom he had enjoyed sex? It may have been suggested generally in public, while a blind eye was turned in reality.
You seem to have a very idealistic notion of history and of the nature of powerful men. There were plenty of royal ‘bastards’ roaming around. Many were even acknowledged and supported.