Not ALL Catholics.
Some of us believe what is written in the Bible.
The Bible specifically mentions Jesus had brothers and sisters. I first learned this in a Bible study class taught at a Catholic church by a Catholic priest.
The Bible also mentions that Mary was a virgin at the time of her conception of Jesus. But there is absolutely no mention whatsoever that she remained a life long virgin or that her marriage with Joseph was not consummated.
The Lord commands us to be fruitful and multiply. Physical intimacy between a husband and a wife is a precious gift from God, not an act of defilement. There is no reason to believe that this gift was denied to Joseph and Mary. The idea that Mary was always a virgin and there was never the gift of physical intimacy between her and Joseph is strictly conjecture on your part.
The Bible specifically mentions Jesus had brothers and sisters. >>>
Those were his cousins.