Mary Morstan, or rather, Mary Watson, was a woman to be reckoned with. And when Mary Morstan found out that her husband had been cheating on her with a man, she decided that something was to be done for it.
So she threw on her frilled Victorian dress, hopped on a horse and buggy, and galloped all the way to 221B, Baker Street. She walked up the stairs, put on a most proper, governess' frown, and banged at the unfortunate door.
And who was waiting there but Irene Adler?
Taken aback, Mary Morstan decided that if she could not bash Sherlock Holmes, she may as well make him lose his mistress.
"Did you know," she began coldly, "that John Watson is having an affair with Sherlock Holmes?"
"Yes, I do," Irene Adler replied. Then, walking closer, "And not only this; I know that if John Watson is having an affair with Sherlock Holmes, his wife will surely be at the scene."
Mary Morstan trembled like a tiny sapling in the wind.
"In that case-" here, Irene Adler began to smirk- "with a bit of judgement and calculated timing, one can get what she wants." And she leaned in and kissed Mary Morstan full on her surprised, open lips.
That night, Mary Morstan forgot all about her husband lest she fall prey to hypocrisy.