I think it's possible. I have a number of really good friends of the opposite sex, and I've never really entertained thoughts about anything more. That being said, my girlfriend is the person I would consider my best friend, I suppose this is normal, but she superseded over a different (male) friend who was my 'best friend' before I met my girlfriend.

So, yes and no. Maybe the reason my girlfriend is my best friend is because I'm attracted to her. Maybe the reason I see my female friends strictly as friends is because I'm in a long-term relationship. Tough to nut this one out.