"... It’s the old nurture vs. nature debate..."
There is no conflict between nurture and nature. Nurture is nature.
"Today we know that men’s and women’s brains are not different. Due to neuroplasticity, our environment changes our gray matter."
There is one thing you have overlooked. Experience is delivered to the brain through a body. The body mediates the experience that the environment delivers to the brain. Men's and women's bodies are observably different. Different bodies deliver different experiences of the same environment. Different experiences create different brains.
Therefore, the differences between male and female brains are natural, even if they are produced by experience.
"...this new definition of masculinity..."
Each man should define masculinity for himself. He should set his own standards and strive to meet them. He should seek to become the man he can be proud of.
A man need not consider the expectations of women, particularly those of Feminists, in defining his masculinity. If Feminists consider your expression of masculinity to be toxic, ignore them.
