The coworker who thought she was an opportunity, and the failure to discipline that coworker, definitely is sexism.

The coworker should have been disciplined for mucking with the code out of anger, but as far as being super uncomfortable around her I feel like a healthy warning from his boss saying 'Hey, you're being inappropriate around her' would probably be better in the long run than firing him.

Sounds like mismanagement more than sexism.

Suppose I had a male coworker who had beef with me because his girlfriend broke up with him and started dating me. Suppose he started reverting my commits and interfering with my work in other ways. Essentially, the same situation with the sexes changed around. But the response should be exactly the same.

At that point a manager would need to step in and lay down the law - don't bring your personal conflicts in to work. And if you can't stop, then work somewhere else.

Its not sexism. It could be considered sexual harassment, maybe. Plan old harassment is probably more accurate.

I'm confused. Are you sure you know what sexism means?