You say you haven’t pictured yourself in a romantic relationship with her…so try doing that. See what happens. I think something might be changing there for you, maybe feelings are evolving.
A sexual thought or two can easily happen with friends, but when they turn into fantasies that are enough that you’re significantly aroused by them, to the point of masturbating…well, that’s no longer a platonic relationship.
Fantasies don’t always mean you want to have sex with a person. Ie. fantasizing about something taboo – like sex with your attractive doctor or someone older or your friend’s boyfriend. It’s a hot fantasy because it’s “forbidden”, but you never really would want it to happen in reality.
The reason why I think there’s something more to your fantasies about your best friend is because friends are intimate connections in our lives. They’re people that are not far enough removed from our consciousness to be able to have shallow fantasies about. They’re very much intertwined with our emotions and rooted in a deep bond. So when you start have sexual thoughts about someone close to you like that, someone you deeply care about, then that’s more than just thinking about what the hottie next door would do to you in the pool.
It’s something far more real.
Sumptuous Daydreams 