Hmm...the opposite sex. Growing up as a kid, most all of my friends were guys, and usually not by choice. I had a few spaced apart opposite sex friendships, but mostly guys. The older I got, the more girl friends I had. Now, my closest friends, some of them are girls (I am a guy too for the record). Also, just to throw in a note here, they also talk to me about girl "issues", which I find particularly amazing since I was always under the impression that it was taboo to talk anything of the sort with a guy.
Anyway, point in case, I have guy friends, but in the end, I get along better with girls. There's not so much of the macho BS that comes along with those friendships.