I'd back off and talk to HR about your concerns if there is a HR department. I wouldnt do it in writing either. I'd be making an appt/ having a coffee chat.

If you were going to write something, I'd rewrite it. The language is very passive and gives an impression of being weak and insecure. The use of the terms "girls" also undos what you're trying to achieve. I'd remove all the prefacing about it being none of your business, because you're basically giving him his argument back to you.

If you bring it up with HR or whomever, do it directly. ie, "I notice you have no/few women in the department." leave silence to get them to fill in. "Is it standard recruiting practice to exclude women from the department?" Or you could write "It concerns me how few women are in the department. What is the company's recruitment policy?" "How do we get more women in the department?" etc etc.

I admire what you're trying to do, but you need a more assertive way to go about it. Also, you may want to develop more political savvy. Machiavelli's The Prince is worth studying.