Many Protestants say that Our Lady had other children than Our Lord, and some even base their claims in Mark 6:3.

It is, nevertheless, the worst choice of Bible passages they could ever make. :)

This verse reads in the original Greek (I dont know about the translation you use):

"Isn't He the carpenter, ***THE*** son of Mary, brother of...". There is a masculine singular definite article (in Greek "ho") before the word "son". This verse, as a matter of fact, proves Jesus to be the only (for it is a singular definite verse) son of Mary.

Besides, the word "brothers" doesn't mean they are sons of the Most Holy Virgin Mary. Even if we took it as meaning actual brothers (and it doesn't), they could very well be sons of St. Joseph.

They will see, if you take the dust off your bible, that the so-called "brothers of Jesus", when refered to along with Mary, are never refered to as Her sons. It is always "Mary and the Brethren of Jesus", and never "Mary and Her children".

Check also who was at the foot of the Cross:

Mark 15:40. And there were also women looking on afar off: among

whom was Mary Magdalen and Mary the mother of James the

Less and of Joseph and Salome,

John 19:25. Now there stood by the cross of Jesus, His Mother

and His Mother's sister, Mary of Cleophas, and Mary

Magdalen.

Matthew 27:56. Among whom was Mary Magdalen and Mary the mother of

James and Joseph and the mother of the sons of Zebedee.

Whom do we have at the foot of the Cross?

1 - Mary (refered to by St. John as "His Mother")

2 - Mary Magdalen (refered to by all the Evangelists who gave names to these woman - St. Luke didn't write their names)

3 - Mary of Cleophas (who can be considered to be a sister of the Most Holy Virgin if we take St. John's words with this meaning), who is the mother of James the Less and of Joseph (Mt and Mk tell it) and Salome (the so called "brethren of the Lord" - and Mk 6:3 also refers to Jude and the "sisters" of the Lord, not only Salome, what would suppose an extended family).

Holy Tradition tells us it was St. Jude's marriage that was being celebrated in Cana when, by the request of Our Lady, Jesus performed His first public miracle. It makes a lot of sense if we understand that Her concern about the marriage of Her nephew led Her to request from Her Son this miracle.

4 - The mother of the sons of Zebedee (refered to by Matthew alone). She might be the mother of the other "sisters of the Lord" that not Salome.

Hegesippus wrote in the Second Century that Cleophas was St. Joseph's brother. So the "brethren of the Lord" are his cousins (sons of St. Joseph's brother Cleophas, who married another Mary who may well be, according to St. John, a sister of the Most Holy Virgin. It would give us a double marriage - two brothers getting married to two sisters - that was very much common at the time), and it explains everything (they way they dealt with Our Lord, the fact that they frequently kept company to Our Lady in Her widowhood but nevertheless Jesus trusted Her to St. John - what wouldn't be made if She had any other children...).