I'm an Ordinary Man
Cyril Ornadel; Julie Andrews; Rex Harrison; Robert Coote; Stanley Holloway Lyrics

Well after all, Pickering, I'm an ordinary man, Who desires nothing more than an ordinary chance, to live exactly as he likes, and do precisely what he wants... An average man am I, of no eccentric whim, Who likes to live his life, free of strife, doing whatever he thinks is best, for him, Well... just an ordinary man...
The singer is reaffirming his ordinary nature and desire for a chance to live life on his terms without any unnecessary struggles.

But, Let a woman in your life and your serenity is through, she'll redecorate your home, from the cellar to the dome, and then go on to the enthralling fun of overhauling you...
The arrival of a woman in your life will cause chaos, as she will take over your space and eventually 'remodel' you into what she perceives to be an ideal version of yourself.

Let a woman in your life, and you're up against a wall, make a plan and you will find, that she has something else in mind, and so rather than do either you do something else that neither likes at all
Attempting to plan and execute things with a woman in your life is a futile effort, as she will always have alternate ideas that won't align with yours.

You want to talk of Keats and Milton, she only wants to talk of love, You go to see a play or ballet, and spend it searching for her glove, Let a woman in your life and you invite eternal strife, Let them buy their wedding bands for those anxious little hands... I'd be equally as willing for a dentist to be drilling than to ever let a woman in my life,
When a woman enters your life, it becomes difficult to find common ground or mutual interests, leading to arguments and discomfort. The artist would rather undergo an unpleasant dental procedure than deal with such strife.

I'm a very gentle man, even tempered and good natured who you never hear complain, Who has the milk of human kindness by the quart in every vein, A patient man am I, down to my fingertips, the sort who never could, ever would, let an insulting remark escape his lips Very gentle man...
The artist prides himself on his gentle, patient nature and self-control, choosing not to engage with negativity or insult others.

But, Let a woman in your life, and patience hasn't got a chance, she will beg you for advice, your reply will be concise, and she will listen very nicely, and then go out and do exactly what she wants!!!
Despite the singer's attempts to remain composed, a woman entering his life will test his patience and lead to his advice being ignored in favor of her own desires.

You are a man of grace and polish, who never spoke above a hush, all at once you're using language that would make a sailor blush, Let a woman in your life, and you're plunging in a knife, Let the others of my sex, tie the knot around their necks, I prefer a new edition of the Spanish Inquisition than to ever let a woman in my life
A woman's presence has the power to drastically alter one's demeanor, causing them to behave in ways they usually wouldn't. The singer would much rather face torture than have a woman in his life.

I'm a quiet living man, who prefers to spend the evening in the silence of his room, who likes an atmosphere as restful as an undiscovered tomb, A pensive man am I, of philosophical joys, who likes to meditate, contemplate, far for humanities mad inhuman noise, Quiet living man....
The artist is an introverted, intellectual person who enjoys a peaceful, solitary lifestyle free from the chaos of society.

But, let a woman in your life, and your sabbatical is through, in a line that never ends comes an army of her friends, come to jabber and to chatter and to tell her what the matter is with YOU!, she'll have a booming boisterous family, who will descend on you en mass, she'll have a large Wagnerian mother, with a voice that shatters glass, Let a woman in your life, Let a woman in your life, Let a woman in your life I shall never let a woman in my life.
Even the peace and quiet of one's own space is shattered when a woman enters one's life, as she will bring with her a constant stream of friends and family, all with opinions on what is best for the artist. The artist would never allow such chaos into his life.