One an say anything, but one must look at the actions.
Why MUST Mary remain a perpetual virgin?
And evidently you did, without doing your research first. To say
The Catholic church thinks sex is sinfulis so vague and yet sweeping. Do you mean the Church TEACHES that sex is sinful, or SOME members think sex is sinful or what EXACTLY do you mean? DO you have an exact meaning?
Why MUST Mary remain a perpetual virgin?
Well, I would have said that you might ask Jesus, but now I see that asking Jesus for knowledge is compared to consulting a ouija board.
But allow me to suggest an analogy: To stand up to one's mother-in-law and to attack a vicious enemy are both virtuous, both examples of fortitude, but one is higher than the other. Neither (assuming the mother-in-law needs to be stood up to - which, of course, goes without saying) is sinful in itself.
Similarly, chaste intercourse, that is, loving sexual intercourse with one's spouse and open to life and consecrated virginity are both virtuous. But since in heaven "they neither marry nor are given in marriage", as my ouija board says, consecrated virginity is what you might call a "higher" estate, bieng more like the heavenly state, in certain sense. (For example, no mother-in-law.) Not all consecrated virgins are "higher" than chaste spouses, but the situation is "higher". Paul says something about this.
But as to the "MUST", I'm not sure what you mean.
I DO know that Episcopal Bishop Spong of New Jersey or Newark or something levelled pretty much the same charge and with similar arguments. Then he went on to talk about how virtuous homosexual acts are.
Further, it used to be that we required laity not to eat for quite some time before Mass, but we've never said the eating was intrinsically sinful, and haven't been accused of saying it. But for your arguments to stand, you'd have to find us saying the eating is sinful, wouldn't you?
One can SAY anything. To say something truthful, though, is hard.