In order to have an incarnate God, He has to come about in a way that is not the usual reproductive method.
That entails the virgin birth (“with child by the Holy Spirit” rather than by a man).
Perpetual virginity is an extension of the supernatural nature of the birth. If Mary had had other children, skeptical people like you, who reject the virgin birth as it is, would have more “grounds” to doubt that Jesus’ birth was supernatural and miraculous.
I don’t see how this has anything whatever to do with an “anti-sex” message. Sexuality is only tangentially involved insofar as sex is the way that human beings are normally conceived. Since this is a miraculous conception, sex could not be involved.
Your view is like arguing that a person who wants to get to the top of a hill by a method other than walking is “anti-walking.” Does that make any sense? No . . .
Now, how one gets to the virgin birth / perpetual virginity scenario to an imaginary position that all of this somehow is an “anti-sexual” point of view, perhaps you can explain to me. It’s not actually there. It’s merely projected onto the state of affairs by those who already believe that the Church is “against sex” merely (mostly, it seems to me) because it has sensible rules (another whole discussion).
It’s like saying that one is “against beer drinking” because one believes that it is sensible to not drink five bottles of beer and then drive a car. Is that against beer itself or is it
for a sensible use of a substance that alters cognitive abilities?
Now, going beyond Mary and the birth of Jesus, the biblical, Christian view of virginity is that it’s a great state
if one is called to it. Paul teaches (1 Corinthians 7) that all should follow the calling that God gave them. If they are called to be single (which in biblical morality, means celibate), this is good, because (as he says) the single person can give undistracted attention to the Lord, whereas the married person is naturally concerned about wife or husband, too.
Both states are, therefore, exalted. The consecrated virgin is considered to be “married to the Lord”. Marriage is a sacrament. It gives grace to those who are married.
I have found that it’s only in the Catholic Church that a single, celibate person is considered valuable, and not defined merely by the lack of a mate. Every other institution and our society (generally speaking) tend to look down upon single people as freaks, weirdos, oddballs, misfits, incomplete, inadequate, people that don’t fit in.
Protestants and Orthodox insist upon married clergy. Our view (Roman Rite / Western / Latin Catholicism) is that we prefer priests to be among the category of people who have totally devoted themselves to God, as consecrated virgins: a heroic sacrifice made for the sake of God. But Eastern Catholics allow and encourage married priests. This is a matter of discipline, and not dogma (thus, it could change).
Again, that hasn’t the slightest to do with some supposed antipathy to sex. If you want that, you have to go to ancient Gnostic views or Puritanism or Victorianism: that sort of thing.
There is not a single passage in the Bible that states that “marital sex is evil / wicked / bad.” If you’re so sure that Catholicism is “anti-sex” then surely you could produce one or more such passages. But it can’t be done because they don’t exist. And you ought to know that already, since you say you’ve studied the Bible a lot.
Catholic Virginity: Is it Merely an "Anti-Sex" Viewpoint?