Errrrr yeah, you are definitely being unreasonable.
He is 25!!! Why does he have to inform Mummy and Daddy that he wants to propose to his girlfriend? He makes his own decisions and its not for you to decide if he is 'ready' or not.
My DD is 25 and her boyfriend proposed to her before Xmas. My first thoughts were to be excited for her, to say congratulations and to buy them a card and some champagne to celebrate. I am genuinely happy for her. She is happy, so that makes me happy.
I think we have a tendency to worry too much about our kids these days. Let him go and make his own adult decisions, and take his own path in life, and yes, make his own mistakes if necessary. If it ends up being a mistake then let him make it.
Is there another reason you think he should have informed you? Will this impact you in any way?
If you are wondering where they will live together etc, then take him out for a coffee to chat about his news and then just ask him the question in general conversation (not grilling him)
And please don't speak to his girlfriend/fiance! You will come across as batshit crazy and overbearing.