It's not quite clear whether this originally refered to sex between gods or sex between a god and a mortal but modern usage covers both.
Those not of a religious nature are free to replace the god in the scenario with their choice Oscar-winning actor or actress. Oscar nominees don't count, they're demigods not gods.
Theogamy is now rarer than in former times, if better publicised on day-time chat shows and magazines. For example the Zeus's write-up says:
There was hardly a region in the Greek world which did not boast an eponymous hero who was a son born of one of Zeus' love affairs. Similarly, most the great families of legend were connected with Zeus.
While theogamy is often fantasised about the reality is often far from pleasant. Zeus's first wife tried for years to escape him (he eventually devoured her). The Brothers Grimm whose collection of fairy tales included instances of theogamy which had to be rewritten with positive endings. In recent times Diana, Princess of Wales was killed as the result of the fame her theogamy bought upon her.