Not having read the article, I can only guess what the "but" is there for!
Nevertheless, I surmise the author is comparing or contrasting two different kinds of sexual activity that human beings can engage in. The first kind of sexual behavior is perhaps more cerebral or rational than the other kind. Imagine a husband broaching the subject of sex in the following manner:
Sweetheart, we haven't had sex for six weeks now. Don't you think it's about time that we made love? Hmmm, sweet cheeks?
While there may a certain, uh, stirring in his loins, the husband's rational part seems to be guiding his words. There is nothing particularly passionate about the words; on the contrary, they are devoid of passion, even though they might lead to lovemaking, albeit of the going-through-the-motions type.
On the other hand, what if the husband and wife are on vacation, and they just returned to their hotel room after an afternoon at the beach. The husband has had an eyeful of nubile young ladies in barely-there bikinis and, well, a man has certain needs . . .. Perhaps his wife is similarly primed, as it were, for a little lovey-dovey, but after a shower and a quick change into some comfortable clothes, hunger pangs take over and they decide to go out for supper at that romantic restaurant on the beach.
After a leisurely dinner accompanied by a little wine, and as the sun is setting, like two love birds they stroll on the beach, hand in hand. As they approach a remote section of the beach where a huge rock formation provides them with a little privacy, they embrace and kiss. One thing leads to another, and before you know it, they start tearing each other's clothes off and start to make love like two twenty-somethings!
A little grunting and groaning ensues and, well, let's just say any rational thought about whether someone might catch them en flagrante couldn't be further from their minds. Now that's when his and her "rational you" leaves the building--takes a powder, and the irrational takes over. In other words, the "out-of breath, out-of-body, can-you-believe-this–is actually-happening kind of sex" occurs.
The Greeks called this kind of love eros, from which we get our word erotic. It's the passionate, animalistic kind of, uh, carnal fellowship. (Is it the French who call the peak of passion un petit mort, or a little death? One has a sort of out-of-body experience that is quite exquisite!)
That's what I think the writer of the Time article is trying to describe. While perhaps less frequent than the first scenario I've described above, the second scenario on the beach is well worth the effort if you can pull it off. It might take a little planning, but on the other hand it's perhaps more fun when there is no planning at all, and the "rational you" takes a powder!
Now if you'll excuse me, I think I need to take a cold shower.