So, just why is it that Catholics are so fixated on Mary’s alleged perpetual virginity to the point of teaching it as a fact, contrary to Scripture?
I think its because they consider sex within marriage dirty, certainly not Godly and Mary would never do that.
Combine that with the surrounding cultural goddess worship, hundreds of years of traditions of men, medival superstitions, biblical neglect and VOILA!
What's been suggested most is that Mary we perpetually a virgin because she was sinless, thus implying that her having sex with her husband would have been a sin.
At least every time I've tried to find out why, the response is always that; that she was sinless.
It fits right in with what you suggested there.