Academic research
A 2006 research paper on IRC attacks found that users with feminine nicknames received 25 times the number of malicious private messages as users with masculine nicknames. The feminine usernames received a total of 100 malicious private messages per day, on average - without saying anything! Ambiguous nicknames received 5 times the number of malicious private messages as masculine nicknames. It's likely that a user with a masculine or ambiguous nickname who reveals they are female in some other way will get similar treatment.
This is important because many geek communities use an IRC channel as a method of communication (especially open source projects), and in particular for people just learning to get involved. Women are more likely to use a feminine nickname on IRC and thus have a higher barrier (25 times the number of malicious private messages) to overcome than men. The only way to avoid this treatment for women is to conceal their gender. This is an additional cost women must bear, as well as denying other women the opportunity for role models.
Examples
These incidents from the Timeline of Sexist Incidents in Geek Communities involved sexism on IRC:
Another example is Rape jokes on IRC used to drive away women.
