I suspect it has to do with the "thought" of Mary having sex..kinnda like thinking about your mother and father
Then there is the issue of a" holy set aside birth canal"
It is important because it is the truth. She could have had other children; that would have been consistent with her married status and certainly no sin. She just didn't, so it is a lie to say that she did.
The truth is that there is no way to know if it is a lie.. This is "taken on faith" in the infallibility of the church alone..
That is true of nearly anything you read in the scripture, too, other than there was a caesar and a temple in Jerusalem. The rest you take on faith from the Church.