First of all I am going to assume when answering this questions that I am only relating to people who are married when I give this counsel. It is wrong outside of marriage. The Bible forbids any form of sexual intercourse (oral, anal, mutural petting, etc) outside of the Biblical Parameters of marriage.
In marriage here is what I would say. If oral sex is wrong, I can think of four possible reasons it would be wrong. I will name them and then I will ask this question. Do those four things exist?
It would be wrong if it were prohibited in the Bible.
It would be wrong if it were unnatural.
It would be wrong if it were unhealthy or, that is, harmful.
It would be wrong if it were unkind.
So let’s take those one at a time.
Number one, I don’t think oral sex is explicitly prohibited in any biblical command. If the Bible pro-scribes it, it would have to be by principle and not by an explicit command.
Number two, is it unnatural? This is a tricky one. The male and female genitals are so clearly made for each other that there is a natural fitness or beauty to it. What about oral sex? Now you might jump to the conclusion and say: Nope, that is not natural, but I am slow to go there because of what the Proverbs and the Song of Solomon say about a wife’s breasts. This is kind of an analogy. So consider this. It seems to me nothing is more natural than a baby snuggling in his mother’s arms drinking at her breast. That is what breasts are. They are designed to feed babies. So is there anything physically natural about a husband’s fascination with his wife’s breasts? Well, you might say no. That is not what breasts are for. But Proverbs 5:19 says: Let her breasts fill you at all times with delight. Be intoxicated always with her love. And Song of Solomon 7:7–8 are even more explicit, speaking of the woman: Your stature is like a palm tree and your breasts are like its clusters. I say I will climb the palm tree and lay hold of its fruit. Oh, may your breasts be like clusters of the vine.
Well, even though there is very little anatomical correlation between a man’s hands or his lips and his wife’s breasts, it surely seems to be, quote, natural, in another way, namely built in delight and desire that God in his Word seems to commend for our marital enjoyment. So I ask: Well, might there be similar desires for oral sex or other kinds of sex? So I doubt that we should put a limit on a married couple based on the claim of it being unnatural. That is risky, but that is where I come down on the naturalness of it.
Here is number three. Is it unhealthy or harmful. Well, it certainly might be if there are any sexually transmitted diseases present. And it could be performed in harmful ways. And so the couple needs to be very honest and caring by not taking risks that would be unloving.
Which leads to the last one, number four: Is it unkind? Now I think this one is probably the one that touches the rawest nerve and the one that has the greatest impact. Will you pressure your spouse for oral sex if he or she finds it unpleasant? If so, then you are unkind. And it is a sin to be unkind. Ephesians 4:2. Be kind to one another. But the key word here is pressure. I know that 1 Corinthians 7:4 says the wife does not have authority over own body, but the husband does. Likewise, the husband does not have authority over his own body, but the wife does. And the context there is sex. So what does that mean practically? Well, it means that both the husband and the wife have the right to say to the other: I would like to blank. And both of them have the right to say: I would rather not blank. And in a good marriage, the biblically beautiful marriage, both of them seek to outdo the other in showing kindness.
So those are my principles, that would guide, I think, the Christian couple in this matter of oral sex.
Help from: Dr. John Piper: This Momentary Marriage and Ask Pastor John podcast as well as Mark & Grace Driscoll's book: Real Marriage.