What does it mean if I have sexual fantasies about girls?

Ok so I am straight. I have never had a girlfriend, never done anything with a girl and I until recently I have never even thought about what it would be like to kiss a girl or anything. But..I have been wondering about that alot lately and I tend to have fantasies about it. I get off more on the fantasy of doing stuff with a girl than a guy lately. What is the deal. I dont think im bisexual but I just dont no? Is that normal for a straight person to have those thoughts about the same sex?