For this quote, two definitions are necessary:
1) sex: ... well, maybe you know this one.
2) subjacency: (noun, linguistics) a restriction on grammatical movement in the sentence that prevents elements moving over more than one boundary, the definition of boundary varying as a parameter from one language to another.
Completely understanding number 2 isn't vital. Number 1... well, Darwin thinks you need to know this one.
Quote: "Subjacency has many virtues, but... it could not have increased the chances of having fruitful sex."
From Lightfoot, D. (1991) "Subjacency and sex". Language and communication 11: 67-69.
This is one of the many reasons why I love linguists.