Morality provides the outer bounds of human activity: it says that action, to be good and truly human, needs to be kept within certain bounds. From one perspective, that means that considerations of morality are not focused on human excellence, but merely on some minimum standard of behavior. Good people are focused on doing good, in the incredible variety of ways that it is possible to pursue the good. They are asking themselves how to be more excellent in the realizing of their human capacities, not what is the minimum they have to do to avoid doing wrong. (Of course, people who act badly often don't intend to do so -- there is a lot of human ignorance and a lot of rationalizing -- but good intentions don't prevent people from going off the road.)
The basic principle that makes a sexual act good is that a husband and wife perform it, loving each other and sensitive to each other's preferences, and completing the act in a way that makes them "one," a distinctive unity: namely, when their act is the act of that unified man-in-woman, by which they could (you could almost say "it" could -- that is, the man-woman unity), in principle (though not always in practice) engender new human life.
This is not a popular way of looking at sex, of course. It excludes most of what has passed for sex in human history and passes for it in today's world: the incredible amount of sex outside of marriage; the typical sex today that is deliberately rendered sterile, incapable of its greatest potential accomplishment; all the sex that is merely two human bodies interacting to maximize the sexual pleasure of the participants in the act. Even among many believing (and conservative) Christians today, what constitutes a genuine act of sexual union has been lost sight of. (This is largely because so many Protestants view Scripture as a treatise with the answers to every moral question, and they therefore consider any sexual act not condemned by Scripture as at least tolerable, including contraception, mutual masturbation of spouses, oral sex, and sometimes even anal sex.)
It is distasteful even to have to mention some forms of physical "union" (in the sense of a part of one person's entering the orifice of another person). While anal sex has always existed, it has become somewhat fashionable in the West these days -- though I suspect that it is popular with a decidedly limited range of people who get more attention than they deserve because they are doing something new and different and provocative. (Even the pleasure of anal sex is, I think, as much from its transgressive quality -- the sense that one is exceeding normal bounds -- as from its physiology.)
Oral sex, on the other hand -- in the sense of completed oral sex, not foreplay -- has probably always been common. But, like anal sex, it doesn't involve genuine personal sexual union. Only penile-vaginal intercourse makes the husband and wife a genuine single unit that does something as a unit: namely, act in a way that has the potential to bring a child into existence. Anal sex and oral sex don't actually make the couple a unit that does anything distinctive -- they are only two bodies interacting.
The irony is that, in an age when there is such an expanding appreciation of what is "natural," for instance, in the food we eat, sex gets ever more un-natural. That is, the very thing that sex is oriented toward by nature -- the union of a man and woman in a reproductive act -- has been technologically eviscerated, in order to distill the pleasure from the act, while straining out the very purpose for which the pleasure exists.