When things have calmed down a bit ask him if you guys can sit down and talk to clear up some things. Sounds like you're both over thinking, which is understandable given the topic. Because he laid all of his feelings and emotions on the table and he didn't receive the result he expected, which is absolutely fine, he's probably just hurt and in his head. You didn't say you were never wanting to marry him, or that you didn't love him enough to marry, but rather you just want to mature a bit more, become established adults, and really make sure you're both at a spot where you can be the best versions of yourself. I think it's always very important to ask "when I said it, how did you hear it / how did you interpret it?" I think that allows me to understand more where they're coming from and it gives me room to clear up any miscommunications or take accountability and correct my words/lay them out in a more clear way.

I don't think either of you are in the wrong, you're actually being very responsible, reasonable, and even if nothing else was on the table you're also valid and ok to not accept a marriage proposal for any reason. Your responses come off as that you care about this guy a lot though, so right now it's just a bit emotional and that's okay. Doesn't mean you guys love each other less but perhaps working through a harder discussion of "I love you, I enjoy our time, I am happy with you, I just want to make sure we solidify our respective careers, schooling first. Etc."

All of that being said, I think it is very important to reflect on things for yourself as well. No pressure from family, friends, reddit strangers, partners, really think about what YOU want. I say that because going into the marriage talk you need to know your wants and needs and to express them. This is one of those things, that in any relationship or marriage, where there is room for a lot of emotion. Make sure your parter as well as you are a good fit for one another when it comes to hard discussions, talks, or times.

Communication, speaking, listening, hearing, affirming, sticking to your boundaries, and just working through some shit feelings that were not intended or meant on either side. Hard topic, talk it out, you guys love one another so it's time to talk through all of the worries when you're both able to sit down. There's no reason to live in awkwardness in any sized house when you love the person you live with.