The implication is sex is dirty, otherwise Mariology comes crashing down."
Regarding the implication it could also be implied as a sacrificial act for the love of God. Following your argument we could also derive the same implication from the celibacy of St. John the Baptist, St. Paul and the virginity of Christ himself.
In the end of course as you know, I believe in the perpetual virginity of Mary because the Church tells me so.
This biker dude has some interesting comments on why Mary remained a virgin after the birth of Christ:
”Why did Mary remain a virgin after the birth of Jesus?”
The Church Fathers also had some things to say on this:
But I still think St. Augustine has a good argument with Luke 1:
" "Surely, she would not say, 'How shall this be?' unless she had already vowed herself to God as a virgin . . . If she intended to have intercourse, she wouldn't have asked this question!"--Holy Virginity (4, 4)
Sigh...
Matthew 13:55-56
Is not this the carpenter's son? is not his mother called Mary? and his brethren, James, and Joses, and Simon, and Judas?
And his sisters, are they not all with us? Whence then hath this man all these things?