With the word 'frat', it's just an abbreviation of fraternity. The analogies used to say that the act of abbreviating it (country and mother) really have no base. It all boils down to a respect issue. I know that any time I go into a fraternity's meeting to speak on anything, I always greet them with "hello gentlemen". Regardless of my having a great rapport with the group, I will always refer to them as gentlemen in a formal context. This may just be me, seeing as I tend to refer to my sisters, and other sorority members, as ladies when I see a group of them anywhere. It's something that I have ingrained into my daily vernacular and, as such, I don't give it a second thought. It's all about respect.
And now I'm done with my babbling.

Have a good one all.