Alan: In both Spanish and English, the Latin root of "ilusión/illusion" is "ludere" meaning "to play."
In Spanish, the chief meaning of ilusión is "hope" or "hopeful anticipation."
The Hindu concept of "lila" -- the notion that phenomenal reality is "divine play" -- enables us to see how "the attractive appearance of humans-in-their-prime" is a playful, even a "trickster," moment that inevitably gives way to realities that are dauntingly difficult to "play" with, realities that are much less "inviting" than the superficial illusion that once provoked so many professions of "eternal love."
The way of the flesh is downhill.
The sexual drive that insures survival of the species depends on an illusory moment so blindingly passionate that we throw ourselves into "the heat of the moment" ignorant of what is demonstrably (and enduringly) "on the way."
"Moderns" often think they control the "use" of sex.
Make no mistake: sex - in its essence - uses us.
We are all controlled -- "manipulated" is not too strong a word -- by the evolutionary impulse of illusory sexual passion (which, in certain blessed instances, coincides with the other-centered commitments of Love).
Despite the utilitarian basis of transitory sexual attraction, it is often a very good thing (at least if we posit happiness as an overarching human goal) to feel that we are "part of it all" and sex -- despite its frequent downside -- is the most dependable way to glimpse The Oneness.