In my head there are two scenarios here.
One in which you TALK about babies and stuff and one were there is an essay on your perfect future.

The first one shouldn't scare away if it isnt a "and YOU HAVE TO BE THAT MAN; BE IT NOW!!". On the contrary, if you think your relationship is deepening, than it's a healthy thing to check if you have similar visions for your future live. If a guy is "scared away" from this, then I think it's a good thing (Imagine beeing together with someone you cannot talk about stuff that is important to you - and your future together)

The essay version is creepy.
But in your case: Why the hell was he reading your diary? I mean it would be really creepy if you gave this text to him (In this point I'm with him) but if this is something you wrote private for yourself then he is just an asshole that needs to learn what boundaries are and you have every right to write down what floats your mind if you like to.