Hobbit on Quest (he/him)
Every group has the right to screen its membership and decide whether you have a right to participate socially, and even if you deny that they have that right it doesn't change the reality of the fact that they do behave this way - right or not. This is true of cheerleaders, Hassidic Jews, feminists, communists, conservatives, jocks, construction workers, goths, etc. You might get more harassment joining a construction crew as a woman, but harassment wouldn't be exclusive to you being a woman. Believe me, it was a major obstacle for me to convince them an 'egghead' belonged in such a group.
Groups may do this, but not every group has the right to do it beyond their own little social clique. That's affording the practice a legitimacy that doesn't exist. That groups do this broadly doesn't make it a right at all. They certainly don't have the right to do it at a convention or on a web site that others have organized and are determined to make open and inviting.
And you really think that someone uncomfortable with your entry into a social setting is going to be more likely to admit defeat and cease being hostile if you .... what? Hold a rally? Call for diversity training? Angrily denounce his sexism? Challenge him for membership in the group? Threaten to exclude him? Appeal to the other members of the group for sensitivity? What do you think this is Survivor: GenCon? You really think even the sympathetic people of a group, the non-sexist people of the group, are going to be really sympathetic if you try to rally them against their friend on account of his sexism? You think this is a more workable solution than showing what you have in common? You think that's a better solution than showing you can't be put at unease and that your are good natured regardless of whether you are treated poorly? You think that's a better solution than showing you are emotionally tough and that you can give as good as you get? Because I promise you it's not, whether we are talking entry into a gaming group or a business situation or a position of leadership.
In the public venues in which this occurs, there are tools available that should be used including bringing in con security or other event organizers. And I, for one, am determined to do so when some cretin tries to alpha geek either my wife or daughter because they're not geeky enough for him.
Private attitudes are private attitudes. But public behavior is public behavior and can, and should, be sanctioned appropriately.