Let us assume that humans of yesteryear were much the same as those of today.
Teenagers are near always randy bastards, and will push the limits of anything they aren't supposed to do, even today they'll fool around as far as they can without crossing that dick in vagina line. The consequences back then were far more severe, yet somehow in 2000 years, parents haven't really grasped, that won't stop the determined, it'll just make them that more fucking cautious.
So consider this, two youths of equal indignity, hot bothered and groping. She wants it, he wants it, but she doesn't want to get rocked to death, and he doesn't want his ass handed to him by her father. So they have as much fun as they can, she gets him near the finish line, he dips the tip in as far as he dares, and boosh. Or hell, maybe he wasn't even that noble. Was gonna just push through anyway but suffered, as is common, a premature blastoff before he could get all the way in.
Either way, there's a hymen still there, and there's a load close enough to where it needs to be. Remember, even the pull out method can lead to a surprise, it only takes a little in the right place. Yes the odds to this scenario are very long, but, if there is an actual chance of something happening, give it enough repetitions and probability always rises to the value of one.
And hey, when something very, very unlikely happens, (like, being the winner of a big lotto jackpot) isn't it common to call it... a miracle?
(tbh, I'm rather surprised a pair of people in the past 50 years HASN'T sussed this shit out and exploited it to try and get some 15 minutes "look, a pregnant virgin! no test tubes or vaginoplasty either!". Hell, even with those two things in mind, there's still probably enough self-deceiving yobs around to make it work.)