That's not what we "freak" about.
If the Romanist dogma was simply that Mary, after being conceived, was thence Regenerated in the womb -- well, it still would be a Biblically-unsupported dogma, but no more objectionable (in theory) than the Biblically-supported case of John the Baptist.
The problem is, Romanism teaches the immaculate conception of Mary. That's rather a different ball of wax, from being conceived in inquity like all children of Adam and thenceforth Regenerated in the womb prior to birth.
Sure it's different, but once you acknowledge that God can "save" one from sin without any prior action required on one's part, the idea of creating one without sin in the first place, as a special grace, is not that far a drive.
SD