Mortal incarnate life on earth is when sexual desire emerges (our pre-mortal life has the sexes but, I assume, not sexual desire).
That is why sex is so important to us - it is new, it hits our immortal souls like a ton of bricks! It is a major difference from everything we have experienced in our pre-existent eternity.
Sex is the essential middle term between marriage and family; and as such part of our primary sub-creative experience.
No wonder sex is so important to us!