Why does the Talmud assume a woman cannot get pregnant on the first act of intercourse?

Question

There is a rule in the Gemara (Yevamot 34a) that אין אשה מתעברת בביאה ראשונה. The question is why the Talmud assumes that a virgin cannot conceive on her first act of sexual intercourse if the hymen is actually penetrable and modern-day observations have proven that a women can indeed conceive then?

Answer 1

I wonder if it might just be a matter of timing. The egg lives for only a maximum of 24 hours or so after ovulation, and sperm also are fertile for only a couple of days. Now, a woman might get married, and have her first act of intercourse with her husband, anytime during the tahor part of her cycle (and, according to the original Torah law, that would be 3 out of every 4 weeks), so the chance of this occurring close to her ovulation would be relatively low. During the rest of her married life, on the other hand, there's a much higher chance that they'll have relations at (or close to) her fertile time.

So this could indeed be expressed as the statistical fact that conception after first intercourse is much less likely (although, granted, not impossible).

Answer 2

Rashi (Breshit 19:36) points out that Lot's daughters got pregnant on the first act of intercourse even if it doesn't generally happen. He explained that hey were in control and used a special technique. The Chizkuni and Zekenim meba'alei Hatosfot explained that they removed their "betulim".