For the entire span on humanity, people have been wondering what is sex and what is means to the human psyche. Indeed, you can assay 100 individuals and you're sure to find a different definition amongst each of them as to what is sex. Merriam-Webster defines sexual intercourse as "heterosexual intercourse involving the penetration of the vagina by the penis." If you were looking for a textbook definition, that's about as good as it gets. However, time has shown a change in the definition of what is sex and much like the act itself, the way people view it has changed over time.
For instance, our parents' generation would most likely attach the euphemism of "making love," when talking about what is sex. Ask anyone between the ages of 16 and 35, however, and you might be surprised to hear that sex and love are mutually exclusive. The rise in casual sex has caused a paradigm shift within people in that many no longer see it as an act that must be done under the covers on your wedding night. Indeed, the rise in incidences of sex within music, TV and literature has caused, as some would argue, an almost too-casual view of sexual intercourse where people see it as nothing more special than going on a dinner date. Of course, many circles still find that view to be antithetical to their beliefs and eschew it for something more "traditional," in that sex must only occur between a man and a woman when they have been joined in Holy Matrimony.
This view has been further challenged by the more-open prevalence of homosexual sex within mainstream culture, where the idea of sex can even forego the idea of penetration. Especially among the female homosexual community, where penetration might not even need to occur to have sex, the idea of what is sex takes on a more mental state: That is to say, having sex is more about the joining of two people, not necessarily about the releasing of fluid into a vessel. However, even this idea is hotly debated and, really, there's no true definition as to what is sex. The best idea to figuring out what is sex to simply have an open mind and be welcoming of all walks of life. There's no use in sticking to one definition because, much like human sexuality in and of itself, the basic definition of what is sex has become extremely fluid over time.