Because sex is EEEEEEEEEVIL.
...that and if Catholicism is ever wrong about any one thing, it opens the door to them being wrong about lots of things, and if it’s wrong about lots of things, it could be wrong about salvation, and that means that the money they spent on their scapulars was wasted.
Among other things.
That comes out again and again.
That whole nonsense about Mary being innocent in body and spirit.
So if she had sex, she was not innocent, which means she had sinned, ergo, sex between a married husband and wife is sin.
Oh, is that why Martin Luther believed in the perpetual virginity of Mary?
But wait, he broke his vows, and got married. And had children. So, that doesn't make sense. Or do Lutherans believe his children were also by virgin birth?
Or, most likely, your conclusion is just another non sequitur.