2. It makes the proposal anti-climactic
Ah, the marriage proposal. From time immemorial it's been romanticized as a huge climax in two people's lives -- and most of the romance comes from the idea that the man and woman are entering into a huge new commitment together. A proposal can still be beautiful and touching if you're already living together, but it'll lack a certain gravitas. If you're already engaging in all the intimacy and sacrifice that comes with making a home together, the moment of the big decision has long passed; in a way, your engagement is already over even before rings get involved.