Myth? Fairy tale? I do not think we should call it that.
St John tells us that there were many things Jesus did/said which were not recorded in the scriptures. We are talking about things done more than 2000 years ago. I think people might have talked about these unwritten things for a long time or even written about them in other ways other than the scripture. And passed on through traditions or legends. Paul told his converts to hold fast the traditions (you might call it teachings) which they were taught whether by word of mouth or his epistles. It is very unlikely that everything Paul taught through the spoken word was recorded in the scripture. Some things would have been passed down the line via word of mouth too. Even in this madness of social media, so many things still circulate via word of mouth without entering the net, passed on from mouth to mouth, not to talk about then.
I do not think we should dismiss issues just because they are not written in the bible. The burden on us is to prove all spirits. ---is it in line with the truth of the scripture; is it in line with the nature of God etc. To me, in this case, the answer is yes because as I said before, it is unthinkable that God would manifest himself as man through a sinful body. Even we are made righteous so that the holy Spirit will dwell in us and walk in us to guide us to renew our mind day by day and quicken our mortal body. How can it be wrong to say that God made her pure right from the womb to shield her from the original sin because of her unique mission on earth? This is not out of God's character. He prepares his anointed ones for unique purposes in any way he deems fit. e.g. John the Baptist etc.
I believe it. I am not saying you or anyone else should believe it too But the least we can do is not to twist other people's doctrine or even the scripture in order to serve our purpose, though, I recognize that sometimes, we do this for lack of knowledge, out of the integrity of our heart in a bid to defend the gospel. Maybe, "to twist" is not the right word. Only God knows the heart. Provided we are persuaded in our heart that we are not saying things out of grudges, hatred, animosity or malignity, it is all good and well.
To digress a bit, I think that original sin is passed on by means of reproduction (sexual), hence, the need for her to be made pure right from the womb. This purity was preserved to conceive Jesus. Recall she conceived Jesus being a virgin, therefore, her purity was intact.
Jesus is the central figure in the gospel and not his mother, so, it is expected that revelation of his being would take the focal point of discussion/teaching and not his mother's.