This article says a study was done and it found that women have a clear preference for men with low, deep voices. Women were shown pictures of different objects and listened to the objects name being said with high or low pitched male or female voices. When the voices were low pitched male voices, the objects name was remembered more often.
Mate choice effects things such as pregnancy, and a low male voice may be used as an indicator of a healthy male. The deep voice is a sign of masculinity. I think this probably goes back many years ago because I don't think people consciously think about a person's voice pitch before they start dating. It's probably something that women do without even realizing they are doing it.
The researchers are going to further look in to this study. Now, they are going to focus on how menstrual cycle and geographical location affect a women's preference.
Though the article said the researchers suggested not trying to manipulate your voice pitch as that may backfire, I found this guide on how to make your voice deeper for all the males out there.