I don't feel any obligation to believe something just because a Reformer believed it. Even if you're correct and most or all reformers believed that Mary had no other children, I doubt any held this belief is essential to salvation.
Having the burden of proof isn't too big a problem. With numerous references to Jesus' siblings, any natural reading of Scripture without preconceived notions would cause the reader to believe that Jesus had siblings. It seems that the Catholic position is the one that has to keep explaining away Biblical passages.
I think the word feel sums it up nicely. It seems to me that the modernist view that Mary had other natural children is a primarily a natural consequence of a reconstructionist type of Christianity that rejects almost two millenia of sacred tradition from both the holy catholic apostolic church and all the reformed faith communities. I see it as a continual rebellion against any spiritual authority other than the individual's. This would then always cause problems in the faith communities where the leadership tries to establish new interpretative traditions and the rank and file members (although antimembers may be a better term) are constantly shrugging off any tradition for autonomy and free expression. It almost reminds me of the Western music scene.