I can only give the perspective of what I think when I see parents of only children... I don't think they are "half" parents or look down on them or anything but I do sometimes wonder if they had an only child by choice or circumstance
And therein lies the assumption that there is something odd/wrong about only having one child, that there must be a reason for it.
I have one child, by circumstance. I would have loved more. But I don't look at parents with 2 or more kids and wonder, did they choose to have that many or was one of them an 'oops'. It bugs me when people question why I have only had one; I don't ask them why they had two or more. A parent is a parent, regardless of whether you have one, a dozen or look after a child not born of you.
OP, I think I understand what you are saying. I have received a few 'oh, it must be easy with just one' from some I meet. I think I was far more tolerant of these comments when I was younger, I get shirty now that I am old and grumpy lol.