Final answer:
The word that Foster is referring to when he says, 'If they write about sex and mean strictly sex, we have a word for that' is sexuality.
Step-by-step explanation:
The word that Foster is referring to when he says, "If they write about sex and mean strictly sex, we have a word for that" is sexuality. The word "sex" can have multiple meanings, but when someone wants to specifically address the topic of sex, they use the word "sexuality".