Can a girl get pregnant if I ejaculate in her mouth?
To answer your question well, we need to back up a bit. A girl gets pregnant when a sperm from a boy fertilizes an egg from the girl. Your sperm is present in the semen you ejaculate. Each ejaculation contains about a half-billion sperm, but it takes only one being in the right place and right time to get a girl pregnant. Given that you release so many, it should be apparent that the odds are not real favorable to the sperm.
A girl releases one egg per month from her ovaries. For fertilization to occur, a sperm has to find the egg in the first 24-hours after the egg is released. Sperm have a chemical sensor that tells it which way to the egg and it has a tail that causes it to swim at a good clip. The path to egg is from the vagina, through the cervix, then the uterus, and then the fallopian tubes.
So, in one sense, a girl cannot get pregnant from having semen in her mouth. There is not path from there to the fallopian tubes. However, couples fooling around with fornication (having sex when you are not married) are not often careful. They get caught up in the moment and forget that every drop of semen contains some sperm and the sperm don't care how they get to the vagina. Thus a careless touch of the semen followed by a touch of her genitals will transfer sperm to where they can reach the fallopian tubes. A common way this happens is snuggling after sex while still naked. Semen still drips out of the penis after ejaculation and the penis ends up near the vagina. The odds for pregnancy are extremely low, but they are definitely not zero.
But what is forgotten is that pregnancy is not the only risk of fornication. "Wounds and dishonor he will get, and his reproach will not be wiped away" (Proverbs 6:33). Oral sex is still a form of sex and it easily transmits diseases. In some ways, the odds are actually higher than intercourse because the teeth can make microscopic scratches on the penis, which allows germs to lodge in the skin or reach the blood vessels. The odds of getting an STD from oral sex is very high.
But most important is to realize that you will be sinning. You are ejaculating with another person, and that is fornication. "Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived. Neither fornicators, nor idolaters, nor adulterers, nor homosexuals, nor sodomites, nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners will inherit the kingdom of God" (I Corinthians 6:9-10). You also have to expose your genitals for sexual reasons, which is also a sin. You are sexually touching another person and that too is condemned. "Now concerning the things of which you wrote to me: It is good for a man not to touch a woman" (I Corinthians 7:1). The behavior leading up to your ejaculation falls under the category of lewdness and I guarantee that your thoughts aren't pure either, which is lust. "Let us walk properly, as in the day, not in revelry and drunkenness, not in lewdness and lust, not in strife and envy. But put on the Lord Jesus Christ, and make no provision for the flesh, to fulfill its lusts" (Romans 13:13-14).
Sex, in all its forms, belongs only in marriage. "Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge" (Hebrews 13:4). When you follow God's laws, you won't have to worry about unwanted pregnancies and sexually transmitted diseases.