I do not believe any of the Reformers held Mary to be sinless (i.e., superhuman). Some of them, like Luther and Calvin, held to the extant view of Mary's virginity, and that the "brethren" of Jesus were cousins or some relative other than half-brothers.
Luther, for example, made rash statements such as, "Christ, we believe, came forth from a womb left perfectly intact." A bloodless birth?? Perhaps, too, a bloodless circumcision eight days later. The problem is, when you study the Reformers you find no Scriptural support for their wild suggestions. They were men, prone to error, as all men are. Let's not make them out to be protestant popes. That's why we can only trust Scripture alone as the ultimate arbiter of divine truth.
I do not believe they had any good theological reasons for maintaining that view. And remember that they lived long before the extreme beliefs of modern RCism on things like the immaculate conception and assumption were codified by the RCs.
The errors they were dealing with in the church were much more grave than devotion to Mary, although one might argument that Mary devotion was a symptom of the larger problem. They may have a different view if they had witnessed the extremes of Mariology in modern times.