The First Presidency has interpreted oral sex as constituting an unnatural, impure, or unholy practice.
I remember when this statement came out. It was mostly ignored by my liberal, East Coast university ward.
The phrase "unholy, unnatural or impure practice" appears to be an expansion of a warning in the temple ceremony against some fairly venal sins such as "lightmindedness." We are to avoid these venal sins and "every other unholy and impure practice." The context in the temple is not at all related to sexuality, so I have to conclude that the the use in the 1982 letter was a novel one, only coincidentally related to the similar language used in the temple except to give the phrase extra authority by unconscious association with the temple rite. Also, the word "unnatural" is significant addition-- it's a word that is loaded with all kinds of sexual connotations. These days it usually refers to anal sex, but it has also been used to make reference to all sexual acts that don't have the possibility of pregnancy. For example, LDS texts in the 1950s to 1970s referred to the evils of "unnatural" methods of birth control.
Shortly after the First Presidency's letter was received by bishops in 1982, a question about "unholy, unnatural, or impure practices" in the marital bed was added to the list of questions used in temple recommend interviews. There was almost an immediate backlash. The first indication of the backlash was a second letter, just about one year later, from the First Presidency saying that bishops were not to pry into a married couple's sex life. (!) Eventually, by 1986, the question was deleted from the temple recommend interview entirely.
Of course, since that time, the Church has further backed away from its prohibition of oral sex in marriage. The most recent guidance seems only to say that "if you feel guilty enough about it to ask, you shouldn't do it." The most common advice you'll hear from bishops is that if the wife objects to it, a husband doesn't press her. In general, Mormon ideas of acceptable sexuality stick to a narrower range than the mainstream culture. For the most part, heterosexual anal sex, the use of pornography as a couple, role play or any kind fetish or kink are pretty much off limits for Mormons. Sex toys (for example, a vibrator) seem to be the wild frontier for the most adventurous Mormons, but even then the Relief Society sister with her trusty strap-on and a mischievous gleam in her eye is beyond imagining.
Although the sin it originally named is no longer a sin, the phrase "unholy, unnatural, or impure practice" is still with us. It is now found in section 21.4.5 of the Church's 2010 Handbook 2:
Adultery, fornication, homosexual or lesbian relations, and every other unholy, unnatural, or impure practice are sinful.
What the current phrase means is left to the imagination. You might be tempted to view it as a reinstatement of the old prohibition against oral sex or any other "unnatural" act that doesn't lead to possible pregnancy. However, the idea that sex needs to be procreative to be holy, natural and pure is contradicted by section 21.4.4 of the handbook:
Married couples should also understand that sexual relations within marriage are divinely approved not only for the purposes of procreation, but also as a way of expressing love and strengthening emotional and spiritual bonds between husband and wife.
This all leads me to the problem that Judge Walker addressed in the court ruling that invalidated Prop. 8. Essentially, the problem is that once you define marriage as a loving partnership that includes relational aspects and not just procreation and the control of property through coverture or inheritance, it makes no sense to exclude loving same-sex couples. In other words, when the Church adopts the position that sexual relations are pure, natural and holy on their own when performed by a loving couple who are emotionally and spiritually bound to each other, then it's much harder to argue that same-sex couples who use physical intimacy to strengthen these kinds of bonds are sinful.
(To be continued...)