<human>: This is pretty straightforward:

I've been with the same guy since junior year of high school (9 years); we are each other's first everything. We did long-distance throughout college (no cheating from either party, as far as I know) and had very full, wonderful, independent lives during that time. We both graduated and looked for jobs in the other's city, and I found something where he lives. We moved in together as an intentional step on the road towards marriage. Our relationship is amazing and I am absolutely in love with him. 

Now he is asking when he can propose. He had NO doubts about wanting to marry me whatsoever.

I, on the other hand, am having doubts. Not because of him or because of our relationship, but because part of me just can't shake the idea that you shouldn't marry your first love. He has none of these worries and I think it makes him nervous that I do. He says that he just knows that I am the one for him. I'm just not like that... I really like to think things through, and so my small sample size with respect to relationships is something my brain just can't let go of.

So I'm hoping to broaden the scope by asking the Redditors out there who have experienced this: what do you think? Basically I just want some perspective... I'm wondering if I were twenty years older would I look at this situation and think, "Of course not! Marrying your high school sweetheart is stupid" or "Don't be an idiot and blow a good thing"? Hindsight being 20/20, lend me some of yours!

Can you condense the text into three sentences or less?
<bot>: I am in a great relationship with my high school sweetheart who wants to propose. I have fundamental doubts about marrying your first love, and would like perspective from experienced Redditors on this.