IF the Bible was authored by humans, which I don't agree with, then yes, it's easy to see that those rules were made out of convenience given the circumstances of the time. But IF the Bible was Divinely inspired, perhaps that gets flipped around. Maybe shellfish could make you sick for most of human history because God didn't want you to eat them. Maybe pregnancy caused issues for women with multiple partners because God, who also gave these decrees, didn't design females for multiple sex partners. We have very modern inventions like condoms and advances in food purification, but they don't change the natural moral decree of God, and those inventions are still fallible. The Word of God is not only written on the page, but in our DNA, and when we look at how male and female bodies are different and how pregnancy works, we can see a second witness to the decree of the Creator of the Universe.
When Jesus said "let he who is without sin cast the first stone" in John 8, he was talking about the woman who was "taken in adultery," while the actual Law of God says that the man who adulterates and the woman who commits adultery must always be stoned together. That was probably just a fancy way of saying "don't stone this woman," because the context of his statement was this: Jesus had just got done writing something down on the ground, and what he wrote was probably (IMO) the sexual sins of those men, which spooked them so much they took off running.
John 8 4They say unto him, Master, this woman was taken in adultery, in the very act. 5Now Moses in the law commanded us, that such should be stoned: but what sayest thou? 6This they said, tempting him, that they might have to accuse him. But Jesus stooped down, and with his finger wrote on the ground, as though he heard them not. 7So when they continued asking him, he lifted up himself, and said unto them, He that is without sin among you, let him first cast a stone at her. 8And again he stooped down, and wrote on the ground. 9And they which heard it, being convicted by their own conscience, went out one by one, beginning at the eldest, even unto the last: and Jesus was left alone, and the woman standing in the midst.
Jesus said "he who is without sin among you," meaning it wasn't some kind of giant declaration that altered God's laws on stoning: it was just a command to that group of men, each of whom were trying to do something unlawful and were trying to dig a trap for Jesus, making him either consent to an unlawful execution or make an adulteress go free. He set them straight, because even though he followed the Law and stopped it from happening, he showed them all that they were no better (again, my inference).