|Cornelis van Cleve, Virgin and Child (c. 1550)|
It's a clever argument, but there are at least three alternatives to bear in mind. First, the Psalms are distinct from traditional prophesy (like Isaiah or Ezekiel, e.g.). The present concerns of the Psalmists are blended with foreshadowing of Christ. Psalm 69 is no exception. On one level, this Psalm is about someone who was falsely accused of theft (Psalm 69:4). Plenty of things in Psalm 69 foreshadow Christ, but that doesn’t mean every element is true of Him. For example, Psalm 69:5 says, “You know my folly, O God; my guilt is not hidden from you.” And Christ is sinless, of course. So the first option is simply that v. 8 isn’t strictly Christological.
|Caravaggio, Adoration of the Shepherds (1609)|
Third, even if Psalm 69:8 applies to Christ, and even if the mother is Mary, the other children need not be biological children. Revelation 12:17 specifically says that the Mother of Christ’s children are “those who obey God’s commandments and hold to the testimony of Jesus.”
So the three options are that v. 8 refers to (1) something exclusive to the Psalmist, (2) Israel, and (3) Christians. Of the three, I think (2) is the strongest, but any are possible. In any case, none of this strikes me as a compelling reason to declare the Church wrong (from the early Christians onward) about the perpetual Virginity of Mary.