It's only a catch-22 if you can't let her know you are thinking about marriage before you propose. Just saying that out loud it sounds like such a ridiculous idea: I might want to marry her, but I can't let her know that I'm thinking about it.

You say it's not a sign of a lack of maturity, but it sounds exactly like a lack of maturity to me (an inability to communicate honestly with someone who you want to spend the rest of your life with about really basic stuff).