There are a ton of Greek myths that start with some pretty woman catching the eye of Zeus, the king of the gods. Nymphs, goddesses, girls – Zeus loved them all. So, when we hear of his lust for Alcmene, Heracles' mother, we are far from surprised. Plenty of gods, demigods, heroes, and beautiful maidens entered into the world because Zeus had a thing for some chick.
With Alcmene, Zeus changes up his usual strategy a bit by seducing her in the form of a man, in this case Amphytrion, her husband. Usually, Zeus does his seducing in the form of an animal, or something even more spectacular. With Leda he was a swan, with Europa he was a bull, and with Danae he was a shower of gold. With faithful Alcmene, however, Zeus thinks the best way to pull it off is to disguise himself as her own husband.
In typical Zeus fashion, he disappears after Alcmene becomes pregnant. This allows Hera, his jealous wife, to take out her anger on Alcmene and Heracles. So, not only is Zeus a philanderer he's also a deserter of those he philanders with.
For much more on Zeus, his crazy affairs, and numerous children, click here.