The Drysau Ceudodol Trilogy

Howard, a smart, successful, and self-confident software entrepreneur with a penchant for adventure, and his very insecure girlfriend Julia get lost on an expedition in a vast newly discovered cave system during their vacation and emerge into a mysterious new world.
This world is the domain of women who are vastly superior to their men in every conceivable aspect, thanks to an extremely addictive extract from the fruit of a magical tree that is toxic for men.
The relationship is subjected to brutal challenges after Julia succumbs to the irresistible temptation of joining the ranks of the superior females of this world.
As Julia is torn between the fantastic power and abilities of her new body, and her desire to salvage her strained and grotesquely unequal relationship with Howard, she is tormented by the looming specter of a painful decision. Holding all the power in the relationship, what will she decide? Can her relationship even survive, and in what way?
However, confusing turns of fate conspire to leave the couple with little control over their choices, and they will have to learn how to arrange themselves with the indelible consequences of Julia's miraculous transformation.