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It's not logical. If you love each other and plan to be together for the rest of your lives then you're ready to marry and move in. If you're not ready to marry and move in then you're not sure that you love each other enough and can be together for the rest of your lives. Everything else is bs.
"We're not ready for the next step yet" is just a simpler way of saying "We're happy where this relationship is right now but we're not ready for a deeper level of commitment". There aren't actually any "steps" it's just an easier way of explaining their relationship.
I love my boyfriend and we plan to be together for the rest of our lives but I wouldn't have moved in together after our first date because we weren't ready. I could also say we weren't ready to take that step and it means exactly the same thing