Male and female are biological terms to describe the dual sexual genders that are prevalent in nature.
But man and woman? Those are terms to describe a state: a girl becomes a woman, a boy becomes a man. What ever that line in the sand is, how ever you want to measure it, what is important that a line exists.
I don't think this will happen in our lifetime, but it may slowly start to become over the next hundred years.
What am I writing about? The use of male and female will remain as gender classifications and the terms man and woman to become primarily used for sexual orientation. You can never stop being male, you cannot remove that X and turn it into a Y. But, you may know that you're a woman.
Or you were born as a female and know you're sexual orientation is that of a man. You can't change your gender, and your orientation is not based upon the DNA giving you swinging bits up high or down low. Your gender is based upon something that LITERALLY to change from one to the other: every single cell and every single strand of DNA in your body must be changed. And not only just a switching of the order of TCGA, but making it so when the individual cell dies it is replaced by a cell with the proper DNA sequence to match the rest of your body. (and we won't even try to pretend we could get DNA manipulation down before words will change meanings)
Sexual orientation is somewhat of a topic in general conversation, or at least is a subject involved in many topics. Today we use words such as "gay" "bisexual" "straight" "transgender"
They are fixed in the current paradigm of male=man and female=woman. Don't believe me? I just used the term "transgender" as an example of sexual orientation words. Orientation =gender in the common mind. But, we have so many new classifications of orientation since the realization by enough people that we are individuals and as such have different tastes in books, movies, exercise and yes: sexual partners.
And unless we stop advancing into such a liberal society, those classifications will increase and what makes a boy a man, or a girl a man, will soon become more muddled then it already is. What man and woman will mean exactly in 100 or 200 or even 1000 years is nothing I can say, but as a scientist I know male and female can never change due to the rigidity of DNA in creating all the fun parts of the human body.
This is what I think about when gays tell me that I am still in denial when I say I am bi... You gays are a bunch of dumb males, and sometimes you act like
Note: Since I am updating this at an unprecedented rate recently (twice this year!) I should let you know I am hanging in the woods for the next 6 weeks... so maybe I will have something good to post then.