The definition of sex is surprisingly hard to pin down. There are a few common tropes such as that which produces orgasm or that which is pleasurable. But neither of these will do because sex can be sans orgasm and pleasure can be induced in other ways. One may say it’s the insertion of a penis in certain orifices, but of course this won’t do either for obvious reasons. Consent is also insufficient.
The traditional western definition is the coming together of two distinct individual organisms as one organism for the production of a new life. However, this takes something like a Platonic or Aristotelian realism and a fundamentally teleological view of nature for granted. This just begs the question: can a society that is essentially naturalistic (i.e. no forms and no teleology) produce a meaningful definition of sex?
The naturalist position is the reverse of the realist position. For the realist, the man as substance is both more real than and is the final cause of his various accidents (such as his appendix for example). This view can most simply be thought of as wholes causing parts. For the naturalist-cum-atomist, atoms are the substance and man is the accident – parts causing wholes. This is the problem of the one or the many. When the naturalist view is fleshed out, not only is the definition of sex problematic but also the principles by which distinct individuals (which are necessary for sexual intercourse) have their being.
Therein is one of the potential sources for confusion in the modern debate regarding sex and marriage. The cart has gone well and truly before the horse; the question of the morality of sex must be subsequent to inquiry into the nature of sex itself. We don’t know what to do with sex and by extension marriage, simply because as a society the west has no understanding of what these things are.