Normally I wouldn't recommend this tactic, (some people say it undermines the validity of the feedback) but in this case I think you might apply a 'negative sandwich' approach, whereby you sandwich your negative statement between two positive observations.
For example: 'I know we're lucky because we found love so early in life and some people never do! Of course for me this means, that I may not be emotionally ready for the things that a 6 year relationship might normally be, like babies. But I know that when the time comes that we are both ready for that, you are going to be a great father, because you are a great man and I am very happy with this life we've got right now'
The good ole negative sandwich!
P.S. I saw ABSOLUTELY no indication that you are a stupid 21 year old and I really resent that anyone would say that to you in a place that you came to for honest input. You sound perfectly rational to me.
Good Luck!
“Love is the ultimate outlaw. It just won't adhere to any rules. The most any of us can do is to sign on as its accomplice. Instead of vowing to honor and obey, maybe we should swear to aid and abet. That would mean that security is out of the question. The words "make" and "stay" become inappropriate. My love for you has no strings attached. I love you for free.”
― Tom Robbins, Still Life with Woodpecker