Long term boyfriend looking up ex girlfriend on facebook!?

[confused]

I know this may seem dumb but I need some advice on how to deal with this.

I have been with my boyfriend for 5 years planning our marriage now. he is my first.

from the beginning I knew he had an ex. nothing special from what he told me.

now after 5 years I found out he had another ex. we share the same computer and I went to the history to check the name of an old friends facebook I saw the other day. I came across a name of a girl I didn't know looked at many times in the history. I went on his facebook (we have each other passes, I have never went in through, this was my first time) I typed her name and it came up first in the list. that means he looks at it quite a bit because they aren't even friends on facebook.

I asked him. he told me it was an ex. he dated for month nothing special. he told me he never told me about her because it was kinda of a fling. she was thinking of her ex when she was with him so he dumped her.

I don't understand that if it was nothing special why the need to look at her facebook page so often? why did he not tell me about her. What was there to hide?

I guess I feel threatened because she lives close to us and in a way i guess he still thinks of her by looking at her facebook.

btw he has no contact with her.

Why the need to check her facebook so often? Once or twice I can understand, but checking up on her once in a while. I don't get it! Help!