A talk at a conference showing girls in bikinis. An API presentation from a sponsor featuring ladies in bras. A demo at a hack day with a slide of women in underwear. A business model canvas workshop using a strip club as an example to illustrate the tool.

These are just a few examples of casual sexism I've experience at (tech) events. It's common for at least part of the audience to react with laughter - sexism is entertainment. I've observed that the photo, comment or story gets laughs from the audience, gets attention. No wonder it feels like it's ok.

When I was younger, I just swallowed my anger. It's hard to say something against a joke; I was worried I would be seen as the too easily offended feminist who can't take a joke. I didn't want to be the old lady telling the kids to stop their ball game. Using sexism in a humorous way makes it even harder to have the courage to speak up.

These days, I feed back to the person who made the joke. At events, I tweet and include the hashtag. Whenever I've approached a presenter and explained how this made me feel as a woman in the audience, I got an apology with the words: "It was meant to be a joke, I didn't mean any harm." Most of the guys were genuinely sorry and glad I made them aware of the damage they were doing.
Speaking up encourages other women to do the same; I've had tweets and emails of ladies saying they felt the same, thanking me for addressing the issue, or sharing their own story and how they tackled casual sexism. I'm so happy to see more ladies at tech and startup events. The first step to change the culture is to be there, and to speak up. We need more people to say that this is not ok.

If you attend an event and encounter sexism, speak up. Tell the speaker. Tell the organiser.
We, as an audience, should not reward these 'jokes'. They're not funny, they're sexist. We should all make a conscious effort to discourage this behaviour, and stop laughing.