This is an interesting point. I find it's often easy to be casual friends with a guy, because all we need is a common interest or two. Like when I used to be in bowling leagues, I could be friends with a guy just because we're on the same bowling team. We only meet up at the bowling alley, and often times, we're just talking about bowling. Women's friendships seem to be more about conversation and less about activity, and so it's harder for me to keep the friendship just casual. At some point, a line is often crossed and attraction develops.