To piggyback of OP's question, what if a noblewoman married a commoner?

First, it was the father's social status which determined the social status of the children. A nobleman who married beneath his station might have been ostracised by his family but his eldest son would inherit the title and the other sons may well have inherited titles as well.

Second, it wasn't unheard of for noblemen to marry commoners. One of the great impacts on English society in the 18th century and on into the 19th was the marriage of sons and particularly daughters of extremely wealthy commoners with members of noble but horribly impoverished families.

Third, however, we have to take a look at the definition of 'noble'. I've been writing as if 'noble' meant 'titled', but it doesn't really. If you were a Baronet, you would have 'Sir' in front of your name and your wife would be 'Lady...' But that wouldn't make you a nobleman. For that, you need to be a Duke, Earl, Marquess, Baron, Viscount - the sort of title and lineage that would give you a seat in the House of Lords.

Now, very few of those married commoners. But they married daughters of mere Baronets often.

I would also like to ask, is this question's hypothetical feasible for its time or is it farfetched? Would a member of the nobility ever marry a lesser man or woman? If so, how frequent was this occurence?

While the other posters make good points, none of them have actually given an example of it happening.

This example comes from 18th Century France instead of England, but close enough. I give you the Madame Du Barry. Born of the lowest birth (the bastard child of a seamstress), grew up as a street peddlar and hairdresser's assistant, then became a prostitute in a casino, and finally a high priced courtesan to the French nobility.

She captured the eye of the king (Louis XV), but because of her common origins, couldn't qualify to be his offical mistress, which was a prestigious court position similar to an ambassador plenipotentiary.

So they just forged a birth certificate showing she was of noble birth, married her to a count, and moved her in with the king, but most of the nobility refused to have anything to do with her because of her low birth, and nobody was willing to sponsor her entrance into the court.

So they found an old noblewoman who cared more about her debts then her reputation, paid her a ridiculous sum of money, and got Du Barry installed in court. They bribed other noblewomen to be her friend, and I think Du Barry had something of a sad life despite some monumental successes.

They never had children, but French kings typically treated the offspring from their official mistresses very well. Lots of generals and admirals were bastard children.

So there's your example of it happening, and the reaction it caused.

One infamous example was Lavinia Fenton, an actress who eloped with the Duke of Bolton after her successful run in "The Beggar's Opera" in 1728. The Duke was already married but they lived together for years until his wife died. They married in 1751--Fenton thus becoming Duchess of Bolton--but none of their three sons they had out of wedlock, while they entered "respectable" professions (church and military) inherited the title after their father's death. Source: "Mistress Peachum's Pleasure: The Life of Lavinia Fenton, Duchess of Bolton," by Lisa Hilton.

Hey, I just thought of a real example, in England, in the 18th-19th century. It's a famous case. It's Emma, Lady Hamilton. She lived 1765-1815. She was base born, a servant! She ended up becoming the mistress of some wealthy dude, then left him for another wealthy dude, then another. She gained some notoriety by becoming a model for one of the most celebrated painters of the day, George Romney. But she was still the lowest of the low, a kept woman of low morals. The guy she was living with needed to get rid of her, so he shipped her off to his elderly uncle, Sir William Hamilton, to lend her to be his mistress. But they ended up getting married, which was pretty damn shocking. She then became Emma, Lady Hamilton. He was high up in the government, having become a member of the Privy Council, which is kind of like, I guess the American equivalent would be a member of the President's cabinet. Her position was likely eased by her not living in England. While they did visit, they were in Naples most of the time, and the people there weren't as fastidious about her origins. Ultimately, her fame rests in the fact that she became the lover of Admiral Horatio Nelson. In fact this is one of the most acclaimed love stories for the ages. It didn't end well for her, though. While Nelson lived, people were able to stomach her. They couldn't marry because both were already married and divorce was almost impossible. He would have married her if he could have. When Nelson died, she became almost penniless. Anyone who talked to her when she was Nelson's mistress turned their back on her after his death, despite his request. She lost her looks and was mocked, she became an alcoholic and ended up in debtor's prison. She died of an opportunistic infection. The daughter she had with Nelson--his only child--denied Emma was her mother, though was happy to be Nelson's daughter. The daughter (Horatia Nelson), despite illegitimacy, was accepted into society, thanks to the assistance of Nelson's siblings, and to the fame of her father. It was also convenient that her gauche mother was dead. Indeed, there seems to have been a willful effort to pretend that her mother didn't exist.

One good example would be Elizabeth Conyngham, daughter of a rich banker, married an Irish peer and eventually became the mistress of George IV. While she was in her younger years looked down upon, there was never any question of her son not being his father's heir. Of course, when she was with the king, she was VERY accepted :-).

I'd also like to piggyback onto this question and ask if anybody has information on when, how, and how widely morganatic marriage was used?

Piggybacking question:

Aren't all women (excepting the Queen, of course) inherently common because they could only hold title through marriage?

The daughter of an earl might be called "Lady Mary" but that is considered a courtesy title.

Maybe OP just means women of noble birth?