Some preliminary points. Because I am talking primarily about male marital sexuality, I will be confining my discussion of oral foreplay to cunnilingus. And I will only discuss it as a form of foreplay, that is, as preparation for sexual intercourse. I should also say that the nature of the topic and some of the necessary details in discussing it suggest that this section should be read only by married men.
I am not ordinarily a big fan of inventing new words, but I make an exception in the case of cunnilingus. That word sounds so technical and impersonal. There is also no well-known and pleasant verb for the act (hardly anyone uses the archaic word "cunnilingue" and slang like "eating out" is ugly sounding) So I will often refer to oral foreplay with a wife as “glossing.” Reasons: 1) it's approprate because it uses the Greek word for "tongue" and 2) it also has the connotations of “shining” or “glossy,” which is an image that fits nicely with the effect of glossing on a wife.
I want to briefly mention one issue, so that I can put it to the side. Some people (conservative moralists) argue that oral foreplay is "unnatural." I have never seen a persuasive argument, however, that there is a significant moral difference between the finger (or hand) and the tongue, when it comes to foreplay. The idea of foreplay is that both partners, but perhaps especially the wife, need stimulation to prepare their bodies for sexual intercourse, and foreplay is therefore quite natural -- that is, it helps fulfill the natural end of achieving full sexual union. (Nor is there any grounds for saying that "only the minimum necessary amount of foreplay" is justified.) Acts of foreplay (including the use of the tongue or mouth on the sexual organs) are part of the entire experience of sexual delight in marital lovemaking, the achievement of true marital union in sexual intercourse. They are therefore very good in themselves.
What is there to say about a husband using his tongue on his wife's genitals -- glossing her -- in marital intercourse? Frankly, I think it is wonderful.
I don't think we know, or likely ever will know for sure, how many women DON'T regularly achieve orgasm in their marital lovemaking. But the discussion of the issue over a long period of time, and the reported experiences of many women, suggest that orgasm-less sexual intercourse for women is, at the very least, not uncommon.
There is much (endless) debate about the reasons for this. Sometimes it is simple male incompetence, as when a man simply doesn't provide enough time for the women to be aroused before he reaches orgasm and ejaculation. But some women say that, even with considerate and loving spouses, they are unable to achieve orgasm. (In some cases they can't climax at all; more commonly they report not achieving climax regularly or consistently in their marital relations). One claim is that ordinary sexual intercourse, for many women, doesn't provide sufficient direct stimulation to the clitoris to bring about an orgasm. (There is an interesting discussion of scientific studies here.)
This website is not going to describe and evaluate these arguments. What I will do is simply repeat the basic point: it appears to be a fact that a significant number of married women, for whatever reason, don't consistently achieve climax -- the culmination of sexual intercourse, with its full sexual release. This is unfortunate, since the ideal is that both husband and wife achieve complete sexual satisfaction (orgasm) in their marital lovemaking. (Karol Wojtyla argued this in the "sexology" section of his book Love and Responsibility, and it is also discussed well here.)
Glossing is often a wonderfully effective way of providing direct stimulation to the clitoris, which helps women to reach climax. I think it is therefore a wonderful blessing. Of course, as with all things sexual, not every women enjoys receiving oral foreplay -- it is not a "magic bullet" to resolve all problems of "anorgasmia." But, as a general rule, glossing is a powerful means of arousing a woman and preparing her for sexual intercourse.
One practical concern about glossing is that the husband is focusing not on his wife's face, but on her vulva. This might be thought to detract from the personal dimension of the act, since the face is the part of the body where we focus on the person. (If a person were talking to us, it would be a bit weird to focus on the person's arm or hand, rather than her face, and especially the eyes.) And a too-exclusive focus on the genitals could encourage a de-personalized approach to sex.
But eye contact is possible in some glossing positions, and even when it is not (as when the husband's head is facing her feet during glossing), the husband's hands can caress his wife's body and face, in order to reinforce the personal dimension of the foreplay. Moreover, there is sense that you are entering her "hidden depths," and that is very personal. As with so much about sex, the attitude you carry into different aspects of it determines whether the act is personal union or merely physical mechanics to achieve sexual gratification.