Human life has different dimensions. Hindu scripture gives them a specific time of the day: religion (dharma) at dawn, lucrative work (artha) in the daytime, erotic pleasure (kama) in the evening. All civilizations have tried to give each of these a proper place. But some people aren't satisfied with this division, and want to unite and fuse these different dimensions. I think there is no need for this, and that it can't work anyway. In particular, the fusion of spirituality and sex is a mirage.
In the "natural healing" centre in the town where I live, courses are being offered in "Tantra". This is advertised as a way to enlightenment through sex. In my Indology studies I have had to spend some time reading the Tantric tradition, and of course it turned out to be rather more complicated than what it is made out to be in the lifestyle magazines. A demythologization is in order, and I'll make a modest beginning here.
Let us clarify first of all that there is nothing mystical about the Sanskrit word tantra. It means "weaving-loom", with warp and woof, hence a multi-dimensional system, something complex and its explanation, hence a manual or simply a "text", a "book". This is the same derivation as that of text from Latin texere, "weave". The Tantra-s are a class of medieval religious texts focusing on ritual and symbolism. In some cases, the sex act is also a symbol-laden ritual, which is why some Hindu and Tibetan gods and goddesses are depicted as copulating, in a dignified seated posture.
In the June 2009 issue of the quarterly EnlightenNext (Dutch edition), the well-known thinker Ken Wilber, who calls himself a "defender of the Dharma" and an "intellectual Samurai", grapples with the issue of sex as a purported way to Enlightenment. With approval, he summarizes the position of the Tantric tradition thus: it says to neo-Platonists and Theravada Buddhists and other ascetic traditions that "you can focus on consciousness and rise to the top of integral unity etc., but you know what... you can also do the same through sex. And sexually it's a lot more fun!"
Oh well, if there's a lady out there who knows the secret of realizing enlightenment through sex, I am willing to learn from/with her. But so far, I don't believe that there really is such a thing as "f...ing for enlightenment", though people are at liberty to try. No dour moral rejection of the whole idea, this, just skeptical that it is even possible. Not on empirical grounds, I can't say I've tried the experiment, but on logical grounds.
In the Buddhist concept of enlightenment or "awakening" (bodhi), the goal of the path is technically defined as "blowing out" (as of a fire), "extinction" (nirvana). This means in particular the extinction of desires ("thirst", trshna), which the Buddha calls the cause of man's ultimate problem, viz. suffering (duhkha). In Upanishadic doctrines of "liberation" (mukti, moksha), the focus is more on the conquest of "ignorance" (avidya), the self-forgetful absorption of the Self in its objects of consciousness; but the need to still the noise of desire is never absent. Enlightenment is, as a minimum, a state of peace, of freedom from desire. It is by definition a state that cannot be bettered by anything that is more desire-fulfilling or, to use Wilber's phrase, "more fun".
Meditation is exclusive of any focus of the attention outside, not even on a dearly loved partner, nor on the sensations accompanying the sex act. In Patanjali's Yoga Sutra, the goal of yoga is "isolation" (kaivalya), viz. of consciousness from its objects, so that consciousness is exclusively focused on (or "resting in") itself. These objects from which consciousness must be turned away are everything that is not the neutral, empty, purely observing state of consciousness itself. They include sensory perceptions, memories, imagination, reasoning, interpersonal concerns, dirty desires as well as noble feelings, anger as well as love. Whatever the value of those things in human life may be, they have by definition no place in meditation leading to enlightenment.
Incidentally, the Sanskrit term kaivalya, "isolation", seems to be etymologically cognate to the Latin words coelebs, whence "celibate". We should not make too much of etymology, and this one should not be taken as proof of any necessary connection between celibacy and enlightenment. Quite a few traditions do think that celibacy is a necessary precondition for serious progress in meditation, others are more generous. At any rate, the term kaivalya in this context does not speak out on the matter. The isolation indicated by it is not that of man from woman, but that of consciousness from its objects. This term merely says that true meditation is a state separate from any and every kind of mental involvement in anything.
After meditation, after "coming down" into ordinary consciousness of and interaction with the world, your experiences may undergo a quality change, and I suppose even sex will not be the same as before. In that sense your sex life may benefit from meditation, but it cannot constitute meditation nor replace it as a method for enlightenment. By all means, make your partner happy, in bed and elsewhere, that's already a mighty contribution to a better world; but please don't delude yourselves that this is enlightenment. The fun of it is good enough in itself and has no need of being labeled "spiritual".
This is really pretty obvious, and it's a bit silly that I have to articulate something so self-evident. Only a spoiled generation like our own could think up this fanciful idea of sex as a way to enlightenment.