It's not about getting offended or not. It's the small assumptions that, when repeated ad infinitum, build up social beliefs and structures that make it harder for certain people to succeed or fit in. The fact that folks on HN or Reddit constantly refer to their anonymous peers with male pronouns might make a female programmer a little less welcome. It may offend her, it may not, but it certainly doesn't help.

"for whatever reason" says it all. If you can't even pretend to care what they think, don't try to pretend you've been welcoming.