Without seeing the article, I would tell you that "sex" and "sexual intercourse" can be used interchangeably. However, it would seem rather formal and awkward to continually refer to it as "sexual intercourse". It is a very clinical term. "Do you want to have sexual intercourse?" would be a terrible way to request it within a couple. As far as a difference between the two terms, "sexual intercourse" includes the act of penetration; however, the term "sex" can refer to the entire experience that includes foreplay, not necessarily ending in intercourse.