Well, you asked, so I'll be honest. Really, at least for me and the girls I know, it's because we can be pretty sure we won't get hit on and will just be enjoyed for who we are, rather than totally sexualized, gawked at, and generally treated like some rare, fascinating specimen from another planet. There are guys I like to ride with (my awesome boyfriend included), and they are the ones that, when they first ran into me on a bike, raised their eyebrows a little, shook it off, and said, "hm, you wanna ride?" I think another factor is that many women didn't grow up riding dirt bikes, etc, or being encouraged to do much in the way of athletic or mechanical endeavors. So, some of us feel a few steps behind when we're getting started, and don't have as much testosterone to get us over the insecurity hump (you know those young guys who just got their first bike? - we don't ususally act like them). Being around other women who don't have such an ego/something to prove thing going on is a lot more reassuring than trying to ride with the typical 23 year old horndog who will be the first to take you up on your offer if you don't specify that you want to ride with women. Hope this helps!