Now before I reveal the sentence below, you must understand the context. The context is one man and one woman permanently joined in marriage, expressing their love in the marital act, which must of its nature be open to the possibility of procreation, and which thereby (and only thereby, only when "sex" lives up to its purpose by existing under those conditions) - which thereby serves as an echo of the love of the Holy Trinity and the nuptial of the Church, which is the Second Coming of Christ. Got it? So if you're a husband and wife giving yourselves fully to one another - including your fertility, which might lead to those delightful surprises called babies, aka new human beings - then, and only then, will the following sentence - THE MEANING OF SEX - apply.
The essence of sex is to be drawn out of oneself, and into another; it's a kind of death to self for the sake of the other, and the new life that such sacrifice brings with it.
Bingo. Done. Send me my check, boys, I'm going home for the night.