My observation is that people never question that part of it, but they question who his father (biologically) was.. because of certain beliefs that mankind inherits original sin from the father, not the mother. so God had to be Jesus' "father" so that he wouldn't be born with a "sin nature" *
however many Catholics believe original sin is from the mother, not the father, and some of them believe both Mary and Mary's mother to have both been sinless in order to weasel their way around that problem.
(in fact that is one of the reasons why my mom left Catholicism and became protestant, but she never fully recovered and passed on a "guilt based" powerless Christianity to her children)
*so where this gets interesting is when you ask the question.. well then how could Jesus have been tempted in all the ways we are, if he didn't have a sin nature.
to answer this, we have to ask "what is a sin nature" and.. well, we don't know.
Paul calls it the flesh which wars against the spirit. but from my point of view, Paul's writings on that subject make more sense to someone with a divided mind because of trauma. one part believes, another part doesn't. one part desires sin, another part doesn't. Jesus didn't have that problem for sure.