So people are like “why isn’t Clary saying yes to Jace?!!!” and these are the theories I have seen (and theorized)
1) Clary is just not ready
Sure, she was a mundane and people these days don’t usually get married young, but she sure has an absurd amount of time to not get ready when she has been with Jace for five years. And judging by her reaction to Jace’s kisses before he proposed, I just don’t see why she wouldn’t say yes just because of it.
2) Clary is pregnant
This is by far one of the most, if not THE most popular theory. It makes sense to a certain extent because well, I mean pregnancy can be a little bit messed up and overwhelming. I can see why she would hesitate to tell Jace this because they are quite young (and knowing Cassie’s characters, they do get abnormal fears). But mind you, “this” means her being (potentially) pregnant, not marriage. And I think Clary wants to tell him no. Think about how absurd it sounds when I say, “Clary doesn’t want to get married because she is pregnant.” It doesn’t actually make a lot of sense.
3) Clary can’t actually... you know, get pregnant
I mean, Shadowhunters are pretty adamant about keeping their bloodline running and stuff. So what if she can’t give birth to a child? To me that sounds fairly reasonable, at least more than the “Clary is pregnant” thing.
4) Clary is... dying?
I mean this sounds really weird but I have seen someone theorizing it and he/she had a point when he/she said that she was being very nostalgic about things (which was not exactly my impression of her in TMI) and the bit about wishing Isabelle keeping Simon safe... as if she might not be able to do it herself. But then again, why would Cassie give her (and the TMI gang) a satisfying closure just to ruin it all over again in a series that is supposed to be centered on Emma and the Blackthorns?...Right?
I guess we will see in Lord of Shadows....