My point of view is that I agree with what you are saying, that the book divides humankind into two very generic categories - men and women. However, I think that this book was written keeping in view certain traits and responses when we talk about relationships, particularly the relationship with the opposite sex. So, if one reads this book with the intention of improving relationships and understanding your partner, then this book serves as a useful tool.
Yeah, all men and women are alike, yet there are some things very common in all men and all women when we talk about relationships.
Thanks for your valuable comments.