Mary asks the angel Gabriel how can she conceive since she does not know man.
If she was to marry Joseph and have relations, wouldn't her question be nonsensical?
The exchange strongly implies Mary's perpetual virginity.
Martin Luther as well as other reformers believed in her perpetual virginity. Besides the constant witness of the Church, Roman and Greek, as well as even some of the early Reformers, how can one living 2000 years later be so sure of himself that he would wish to publicly cast doubt on what had been a constant, universal belief throughout Christendom for 1500 years?
She does not know man when she asks the question. She is free to know man afterwards, isn't she?
I believe in the virgin birth but in no way see this quote as an absolute pledge on her part to never know man.