Sept. 8. Vera, Fulla and Phineas

I wonder at the stark contrast between Phineas' two lovers Vera and Fulla and the reason for the detail that Byatt uses to describe their relationship and the nature of that relationship with Phineas.
Vera, a pale, thin young woman who works as a radiologist, lives in a dark and ordered space and who's passion is the artistic renderings of x-rays. Fulla is bright, voluptuous and fierce and passionate about insects and improving the condition of the planet through a holistic process that would incorporate the roles played by the insect population.
Vera seems to act as the voice of reason, intellect and investigation. Her passions in life, the x-rays, a relative of photographs, which Phineas discovers to be linked rather closely with death, and her investigation into the life of her dead uncle Scholes Destry-Scholes, seem to be the embodiment of Phineas' search for facts and things. Her fragility and her emotional break down after viewing the x-ray of a dying man, could possibly symbolize the frailty of an existence devoted entirely to a life of study of others.
Fulla on the other hand, is the emotional figure, full of life and vibrancy who knows who she is, what she wants and where to go to get it. She is also tied to the planet and has all the characteristics of an earthy creature. Her passion, insects and the identification of their roles in nature, and her physical appearance, a bright cap of bushy hair that seems to defy all constraints, suggest that she symbolizes man's connection to nature. Nature that is always linked to emotion, but solid and unyielding. She ties reason and intellect to nature through application of her studies on insects. 
As Phineas develops his relationship with each he begins to take in the traits of both, or rather to discover those traits within himself. He becomes fuller, and even his complexion and disposition improve. He learns to be invested both with facts and things, but also with their application in the real world. He is no longer concerned with the dead, his biographical subject and all of his biographical subjects have long since passed, but with how to improve the lives of the living, through his work with Fulla and to a certain extent, his work at Puck's Girdle where he helps people find a way to travel on strange paths.
Yet why is it necessary for Phineas to have a physical relationship with these two women at the same time? Socially such things are usually not acceptable and Phineas himself seems slightly embarrassed with the situation. In older texts, such a relationship would develop as a  friendship would result in the same intermingling of perspectives. Why can Phineas not have an intellectual relationship with the women? Is it the time period the book was written in? Such a situation is much more socially acceptable now than it would have been even fifty years ago. Did the author add the relationship in order to attract a larger audience? The subject matter is rather dry after all, perhaps she thought that such a situation would draw in more readers. Also one could argue that Phineas is using the two women in a rather horrible way and the women lose their value as intellectual beings in the process. Personally I find that it detracts from the overall story (Alfred Hitchcock's hidden violence, that your own imagination fills in, is much more frightening than the blood baths that most horror films include).
Perhaps there is a reason for Byatt's decision and I'm just not seeing it.