As a software developer, I think I can tell you what it isn’t: It’s not some he-man sitting in his cubicle saying “huehuehue” as he maniacally smashes the reject button and heads out to catch another beer with the guys.
That’s expensive. Rejecting code costs money. And not just “money” for “corporate” in the abstract, it costs opportunity, slows the team down, requires further communication and checking…. It’s not worth the giggles. There are just fewer female developers, and fewer female developers means less market competition. If you’re a guy who wants to get hired by Facebook, you’d better be really damn good. If you’re a girl who wants to get hired by Facebook, you’d better be really good. People who are really damn good at coding get less code rejected than people who are really good at coding.