Every time I see one of these threads, I'm unable to understand responses such as yours.

Whether he's referring to his girlfriend or women in general, he's perpetuating a stereotype. This has less to do with sexism, and more to do with offensive stereotyping. You could replace the girlfriend example with anything from this page (first link i found) - http://examples.yourdictionary.com/stereotype-examples.html - and it's equally wrong.

He's doing it as a representative of his company, and so Atlassian needs to acknowledge that it's wrong, and they're going to do something about it. An employee made a mistake in a public forum - simple as that.