I read/heard somewhere that a saint (and I apologize, but I cannot remember his name), believed that Our Lord was in the Blessed Mother’s womb one moment and in her arms the next, “as a light passes through a crystal”. That is, He did not need to navigate the birth canal as babies must in order to be delivered (except via C-section, which was not practiced in those days). It would have been a miraculous, painless “delivery”.
I tend to believe that whether this is true or not, her delivery would have been painless, and given the miraculous nature of her pregnancy, a supernatural method of delivery is no less believable. I’m saying this as a scientist-student who is at this moment wondering how the fetal membranes would be expelled or even if they were. Maybe those would have miraculously disappeared too. But I don’t think any of this is that important, and at least during our time here, we really won’t know.
And just because a girl ruptures her hymen horseback riding doesn’t make her any less a virgin. However, being intact is the physical sign of virginity and as such I can see the emphasis on it, especially in those days.
I do not see how it would be possible to deliver a child and physically remain a virgin (hymen intact) in the natural realm. But I am not an OB/GYN and my knowledge of reproduction is mainly limited to livestock. I suppose “technically” you could have a “virgin” ewe if you artifically inseminated her (as a maiden, never bred) laparoscopically (which is how AI is typically done in sheep for reasons that I will not get into here) and then delivered the lambs via C-section. Obviously AI is a grave sin if performed on human beings and I do not know how it is done…but I still think that would violate the spirit of virginity even if the hymen was not ruptured during the process and the child was delivered via C-section.