Cry me a river. It's nobody job to hold your hand and to make you feel welcome. If the default is "white guys hating on everyone else", that would indeed be bad for the environment. But, if the default is "I am here to code, and I really couldn't care why you're here," then it's nobody job but your own to make the environment into what you want it to be.

Response to child: No, by bringing more black girlfriends to a hackathon they would make an environment friendly. By creating a segregated event, they are pretty much saying they can't deal with mixed events, and have to kick everyone else out in order to create an enjoyable environment.

Yes, and a white one at that, but I sit next to and chat with men and women of all races, and I watch the same occur in chairs/tables next to me. I've never seen anything that indicates white men are making others feel unwelcome.

Perception does not indicate reality. If you feel unwelcome and don't participate because of that, there's not much I can do to help you.