I have had anxiety and PND diagnosed but I don't think it's that, I just don't want to. Are you absolutely sure it isn't that?
I only ask because if it wasn't because of the anxiety and PND being a reason for only wanting one, why mention it in your post? I don't mean this in an offensive way so please forgive me.
Tonight, I went to the GP to discuss my fears about a second pregnancy. My DH was adamant that he only wanted one child whereas I have always made it clear that I wanted two children. Only two weeks ago my DH finally opened up and said that the birth of our DS who is now nearly 21 months completely traumatised him (failing placenta, baby not growing, low fluid, rare reaction to indcution resulted in violent contractions, baby in distress, placenta abruption, massive loss of blood for me, uterine tear, baby having suspected infection, severe jaundice!) and has admitted that he would like a second.
When I went to the GP, I asked a question about my fears over rhesus negative and second baby being positive because it was 5 days before I received the anti D again. I mentioned my birth difficulties and the GP asked if it was just the anti D that was worrying me because of me mentioning the other stuff. I paused and thought about it and he was right to question me because deep down the birth trauma was worrying me immensely - I simply do not want a natural birth because of the fears that it could happen again.
My GP was superb and reasurred me a great deal.
I suppose what I am saying is if you REALLY want only one child then that is your decision and nobody elses but you may need to think about why you posted your AIBU post and why you mentioned the anxiety and PND because from the perspective I am seeing this from, this has a greater bearing on your decision than maybe you realise and further down the line you may regret your decision.
It's only been 17 months since you had your little boy so relax, enjoy him growing up and readdress the situation if the need arises later on. As for everyone else, be honest and say that you are happy with the one child you have and that you have no plans to extend the family - you are certainly being honest with that sort of reply.