KyleGoets,
There are different factors to your question. I'll talk about sociological factors if need be, but first let's get this "pitch thing" straight. And oh, I'm a native Japanese speaker, born and bred in greater Tokyo area. Don't know if that qualifies me for anything though.
I used the word "pitch" literally. The frequency with which the air vibrates. And while I don't deny ゆう sounds higher, the pitch of her voice is actually lower than Paris Hilton's, at least in these 2 videos. In the first 15 seconds of the ゆう's interview, the highest note, に in ２食, is a tone called B2, and the lowest point, く in ２食 is B1, both of them can easily be pronounced by any male. Paris is about 3 piano keys higher, for example "good" and "thank you" are around F#3 at the highest point and F#2 at the lower part. F#3 is kind of high for a guy.
The reason ゆう "sounds" higher is because the texture of her voice is more falsetto, and Paris' more hardy. This is due to the nature of both languages. In Japanese, you need to constantly and continuously pronounce voiced phonemes, and your vocal cords aren't contracting very hard, resulting in clearer, but weaker voice. In English, you need to pronounce the stressed vowel, and only that vowel, very hard and clear, and this gives you a bit of "growl" to your voice, kind of like a rock singer. It also goes really low before a voiced sound dies out for a fraction of time, making it sound deeper.
So first of all, Japanese girls "sound" higher if they speak at the same pitch as English speakers do.
And secondly, Japanese is a pitch accented language, which means you can't arbitrary change the pitch like you can in English. American people actually do raise their pitch if they are, say, talking to a customer or being excited. But they only do so at the stressed vowel. Like if you say "Good morning", it's either "Goo" is high and the rest is low, or "mor" is high and the rest is low. But in Japanese, if you want to raise the pitch of "おはようございます" it HAS TO go up at "は" and it HAS TO stay at that exact pitch until you finish saying す. Try this yourself and say "Good morning!" cheerfully as if you are talking to a customer, and try saying "おはようございます" using the same lowest and highest pitch. It will sound strange in Japanese, and it'll be kind of like shouting. So you tend to rise the pitch of the whole sentence a bit in Japanese instead of going very high at the stressed vowel like you do in English. You are going higher in both languages but in different ways.
Now, I'm not saying there isn't any social factors or gender factors or whatever, there are. But you and your friends perceive what is physically lower frequency voice as "high voice", so I just thought this is something you want to take note.
And thank you Nyororin for divulging the girls' secret
That definitely is an important insight.