In regards to "Can a girl get pregnant before her first period?" I really want to say that by reading this, you can no more tell if this is the person who may have sinned or someone who is uneducated and has a child who has been sinned against. Just as it is written, I don't see where the writer of the question is to be accused. If you have not printed this person's entire response to your Scripture, you should only have answered the question in the simplicity it was asked. You are not supposed to judge and you did.
"Judge not, that you be not judged. For with what judgment you judge, you will be judged; and with the measure you use, it will be measured back to you" (Matthew 7:1-2).
I find your anger over my response amusing because in your rebuke you are doing the very thing you are accusing me of doing -- judging another person. Yet, for you, it is fine to do so, but for me, you think it is a sin. Jesus did not say not to judge at all. In fact, in another place, he said, "Do not judge according to appearance, but judge with righteous judgment" (John 7:24). Jesus is warning that when you judge another, you open yourself up to be judged by the same standard.
Now, in the prior question, the boy said, "My friend and I were fooling around and I fingered her." This is not a parent concerned about a child sinned against as you asserted. Nor can we conclude that because a person didn't know that a certain action was sinful that this somehow makes the action not a sin. Under the Law of Moses, "If a person sins, and commits any of these things which are forbidden to be done by the commandments of the LORD, though he does not know it, yet he is guilty and shall bear his iniquity" (Leviticus 5:17). Ignorance of the law doesn't excuse sin. We hold the same principle in our own country. Try telling a police officer, "But I didn't know it was illegal!" and see how far it gets you.
What the boy said he did is called fornication. It is sinful and he needed to know about it. My not saying so would have left him thinking that there was nothing with what he did. Then I would have been in sin.
"Son of man, I have made you a watchman for the house of Israel; therefore hear a word from My mouth, and give them warning from Me: When I say to the wicked, 'You shall surely die,' and you give him no warning, nor speak to warn the wicked from his wicked way, to save his life, that same wicked man shall die in his iniquity; but his blood I will require at your hand. Yet, if you warn the wicked, and he does not turn from his wickedness, nor from his wicked way, he shall die in his iniquity; but you have delivered your soul. Again, when a righteous man turns from his righteousness and commits iniquity, and I lay a stumbling block before him, he shall die; because you did not give him warning, he shall die in his sin, and his righteousness which he has done shall not be remembered; but his blood I will require at your hand. Nevertheless if you warn the righteous man that the righteous should not sin, and he does not sin, he shall surely live because he took warning; also you will have delivered your soul" (Ezekiel 3:17-21).
You, however, want to leave a person sinning in ignorance and me to be sinning against God by not doing my God-given duty. As Paul said, "For do I now persuade men, or God? Or do I seek to please men? For if I still pleased men, I would not be a bondservant of Christ" (Galatians 1:10).
I do hope you will repent of your sin, though.