It is hurtful and stupid if people make rude comments but I think you're being a bit over sensitive because you aren't happy with your family size.
I've only skimmed the OP's responses, so have no idea if that is the case, but I can assure you I had one child by choice, am not remotely sensitive about it (as well as being a fairly upfront kind of person), and I still find the string of comments, head-shaking, tutting, unwanted pity down the years very irritating and presumptuous.
It's particularly annoying when they convert one of DS's good points into a tragic side effect of him being a Lonely Only. Like some stranger will praise his confidence or ability to talk to adults (he's 4)and then shake their head and say 'Oh, it's because he's an only child -- he's only ever around adults, the poor little mite!' Or when someone says he's very imaginative, which is true, and then comes the headshake and 'It's because he's an only child and has no one to play with!'
You'd be amazed what can be turned into a side-effect of him being an only child - the fact that he's very sociable ('He has to be, the poor little man, with no other children at home!'), good at spelling ('Of course he is, he's not competing with anyone for your help with homework!') etc etc.
Sometimes it's funny, to be fair, but it's astonishingly presumptuous. I don't cluck and tut over whether people choose to have enormous families or stay child free, have twenty-year gaps between their children, or have ten months between them. Please extend my only child the same courtesy. I don't actually care that you once knew an only child who turned into a serial killer.
I've posted this on here before many name changes ago, but I found it hard to forgive my MIL when DH and I flew back to our home country to tell PILs and my parents we were having a baby ( I was 40, we'd always planned to remain childfree before deciding it might be interesting, so no one was expecting this) - DH is MIL's youngest of five and golden boy, and still when he told her, her first words were 'An only is a lonely!'