Why do people do oral sex?
The desire for sex is strong in people, but at the same time people don't want the consequence of sex, which is children. Thus, they look for ways to be sexual while lowering the chance of pregnancy. Oral sex is one of those attempts. You'll find people claiming that oral sex isn't really sex because the penis doesn't enter the vagina. But sex is more than just a guy ejaculating into the vagina of a woman. Using such a narrow definition would mean that homosexuals can't have sex, which clearly isn't the case.
A better definition of sex as whatever stimulates another person to orgasm and the use of the mouth can do that. Homosexuals are limited to these non-vaginal sexual acts to get their thrills.
While oral sex does reduce the chance of pregnancy by keeping the penis at a distance from the vagina, it doesn't eliminate it. It just takes one sperm in the right place at the right time to get a girl pregnant. People acting sexual aren't usually careful. Semen can get close to the vagina in a variety of ways and the risk of pregnancy, while small, is always there.
What is overlooked is that oral sex does nothing to prevent the spread of disease. Most sexually transmitted disease involve the exchange of body fluids. That still happens in oral sex.
Oral sex doesn't change the morality of the situation either. Sex between unmarried people is still sinful. It is still a form of fornication. "Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge" (Hebrews 13:4). It still involves lust, which is strongly desiring what is unlawful. It also involves lewdness, which is the acts to stimulate sexual desire in another person. "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). And it involves sexual touching, which is also forbidden in Christianity. "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).