Sex and it's meaning.
Ever insightful and eloquent, Frederica Mathewes-Green wrote an article in the latest Touchstone magazine titled Bodies of Evidence where she explores the meaning of sex.

Whether we attribute extra meaning to humans or not, we are at least animals, sharing this planet with many other kinds of creatures. . . .From that perspective, the “meaning” of sex is pretty obvious. It’s reproduction. . . .But there are some interesting ways that humans are different from other mammals, even from other primates. For us, sex feels good at any time in the fertility cycle. Other mammals mate only during fertile periods. . . .What’s more, researchers suspect that only among humans is the female capable of orgasm. . . .It looks like the “meaning of sex” for humans is something broader than simply reproduction. . . .You can see the same analogy with food. . . .We don’t eat solely for nutrition. Likewise, we don’t have sex solely for reproduction. . . . another way humans are unique. We’re one of the very, very few mammals able to have sex face-to-face. Seeing each other’s faces means something—not just during sex, but all the time. . . .Sex is, if nothing else, about making a connection with another person, and that seems to be something that humans have trouble with.

I recommend reading the whole article. She makes some great insights about sex and at the same time you'll see someone doing really great Natural Theology. At the end, she has a couple of sidebars, one of which is on "Old Married Sex". My favorite lines from there (she's writing about her and her cohorts views as young 60s radicals):

We made fun of old married people, the ones who got hitched, settled down, had kids, had mortgages, and thirty years later were having old-married-people sex with each other. It turns out that, even if you make fun of people like that, you still get old anyway. The alternative is not staying young forever; the alternative is being just as old, and not having formed any lasting relationship, and going to bed most nights by yourself. You’re not having old-married-people sex; you’re not having sex with anybody.