Oh boy, everybody's favorite dirty-old-god is totally up to his usual tricks in this story.
The one thing the king of the gods liked more than bossing all the other gods around was seducing every pretty young thing he could lay his hands on. Seriously, in almost every story Zeus pops up in, he's changing into some weird form—a cloud, a swan, shower of gold—to seduce a girl. So yeah, Io is only one conquest in a long list of conquests. Leda, Europa, Alcmene, Danae, Leto—all these lovely ladies fell prey to the, um, charms of Zeus.
The fact that the Greek king of the gods is such an unrepentant philanderer is kind of interesting by the standards of many modern day religions. What does it say that the big guy in the sky, the dude in charge of everything, doesn't give two flips about stepping out on his wife, Hera? We have to remember that in ancient Greece, Zeus' philandering might just have been a reflection of what was seen as normal by the people who worshipped him.
Are we supposed to forgive Zeus by the end of the story? He does come around, after all, giving in to his wife and pleading on Io's behalf. He could have just gone his own way and forgotten completely about this poor nymph, but he comes to her rescue—even if it was his fault in the first place.
Want to hear more about the king of the gods? Check out his Shmoop files.