Seriously, and please don't take this the wrong way...but I'd be more worried about the fact that you found her diary and read it. I'm not saying parents shouldn't EVER read diaries, go through drawers, school bags, etc. But, unless you suspect something and are having issues with your daughter - I think it's fair to let her have some privacy. Like the other poster mentioned, please don't let her know you read her personal and private thoughts. That could really upset her and drive her from talking to you.
To answer your question...heck yea it's normal! I lived in Colorado for a short time when I was in 2nd grade and I had a crush on a set of twins (Sean and Chad!) By 3rd grade, I had a two "boyfriends" (not at the same time). I even kissed both of them. But, it was innocent curiosity. And, I didn't turn out to be "easy" or anything as I got older - far from it, actually.
At a young age, my mom explained the basics to us. We were always able to go to her for any questions, advice, etc. She never made us feel ashamed or judged. Talk to your daughter about boys. Be open with her. Let her feel comfortable to come to you. Having that type of relationship with my Mom did wonders for both my sister and I.
Good luck!