I'm female and my best friend is male - this has never been an issue for either of us. However, as we spend more time together, I get more comments from other friends about whether or not we're together and how a guy can't be just best friends with an attractive girl. I asked him what he thought and he told me he has no romantic feelings for me at all.
Does this necessarily mean I'm not attractive enough?
When a girl and a guy are friends for a long time, any attration between the two will slowly fade, if there was any to begin with. If you guys spend much time together, he will see you as friend and not as a romantic interest. This doesn't mean that you aren't pretty/attractive enough, it just means that he likes you as a friend (a real good friend!), which is a good thing! I've heard several girls complaining that every man wants to sleep eith them and that no one want to be "just" their friend. You should concider youself lucky :P
No not really. Everybody has different types, I'm sure plenty of other guys will find you very attractive. As for me, I've had some girls think I was ugly as f***/too young looking/not muscular enough, yet I've had others who think I look hot, that I have a good body etc. So don't worry about it.
Nah, sometimes it works out that way. Especially if both parties have a type that is different than what their friend is. For instance, a guy who likes tall athletic blondes may not be that interested in a petite brunette that plays the cello, but that doesn't mean both girls can't be equally attractive. Same as how some girls like beefy jocks and others like thin poetic types, etc. (Cliches, but you get what I'm saying).