They're personalities... I mean everyone is different and no two guys (or girls) have the same personality, but there is a generic difference between how men and women think... (yes, yes, some women think in a more masculine manner, and some guys think in a more feminine manner, I know)
I just relate to guys a lot more than women... they're typically more objective rather than subjective thinkers... don't let emotions cloud their thinking as much, don't fret constantly about what others think about every little thing, are generally more confident and less emotionally dependent, and my interests usually match up with theirs than the typical female...
Of course everyone is different and it can't encompass it all, so its stereotypical in its self, but I don't really care what they look like, or smell like (would prefer not to have to smell their BO, but that's a cleanliness issue), or that they have a penis, (although I admit, I much prefer guys voices to girls), but rather the way I can relate and deal with them because of who they are...
Almost all my friends are guys, and even when there're girls around, we just don't seem able to click... (and this includes older people too, I get along with my grandfather way moreso than grandmothers, in fact both of them irk me to no end, get along with my uncles more than my aunts, cousins.... I don't really get along with at all >.> (probably because they're all so young and bratty and kids and UGH I HATE-scratchthat-DISLIKE CHILDREN)
Its very hard to put into words... I'm completely unable to identify personality traits and explain things... if you were to ask me what my father is like, my mother, my friends... I just wouldn't be able to explain "well he's nice.... and he likes ___ and ___ and ___" so can list interests, but can't really list personality traits... but there is a similarity amongst [most] men, and a similarity amongst [most] women, myself included; so that is what I like....
Otherwise, their voices are certainly more comforting/nicer sounding XD (*generally*)