The issue with this line of reasoning, it seems to me, is that while we may agree on certain axioms and definitions, I don't believe this implies we've created them. We have simply arrived upon certain ideas and agreed upon them. Before man defined what a line was, did the concept not exist? I think it's important to specify that I am not talking about real world approximations to the concept of an ideal line, which undoubtedly existed well beforehand, but rather the concept of an ideal line itself. Is it fair to say that we arrived upon this concept and accepted it, or that we invented it? Indeed, to say we invented it would seem to imply that the definition of a line depends entirely upon the mental capacity of human beings. This might imply by extension, that if our mental processes functioned differently, the definition of a line would change, a notion that seems utterly at odds to with me. We might call it differently, and refer to it in different ways, but the notion of a line remains the same in my opinion irrespective of human existence.
If we examine the other line of reasoning (which I am more inclined to), that we discover mathematics, this would imply that mathematical concepts have an independent exist of their own thereby allowing us to discover them. The immediate question here, is if mathematical concepts have an independent existence, where do they exist? Here we find ourselves returning to platonic ideas: the theory of forms. The theory of forms states that abstract concepts such as mathematical ones have an independent and immaterial existence outside any parameters we associate with the physical world, such as space and time. This is a hard notion to ever accept, and one for which by definition we can never produce empirical evidence. Nonetheless, it works for me.