It's entirely possible for a guy to have a lasting purely-platonic close friendship with a female.

I think it's a potential redflag (that is if are seeking a long-term monagamous relationship) when a girl has mostly guy friends though. If a girl doesn't have many girlfriends but yet many guy friends, something is up. Like adriane said, girls who are like that tend to be attention whores and see potential female friends as threats. Or she may just simply be a slut. Women tend not to like promiscous women because they see promiscous women as competitive threats. When I think back to every girl I've known in my life who fit this profile, she was a slut. I'm willing to accept that there may be exceptions to the rule though. I just haven't seen any in real life yet.