Do you find it weird that Luther believed that Mary was sinless ...?

I do not find it weird at all. Martin Luther had a HUGE Marian devotion. It was only later removed because even HIS followers "protested" against him. The issue with Protestantism is that anyone who disagrees is allowed to break off and start their own church with an even more modified set of teachings. This is why we have 30k+ Protestant denominations today... and still only ONE RCC.