Hi Natacha,
I just now got this email (perhaps it was held for list moderation) but thought I would note that there have been gender-specific events before. I'm aware of the WikiWomen's lunch which seems to be held yearly at Wikimania, and I believe that Wikimedia Mexico has women-only editathons.
As a male I have no objections to gender-specific events happening on occasion. I think that it's fine to have those kinds of sessions so long as they are designed and resourced in ways that are arguably fair and don't suggest favoritism. I would be concerned if such events happened in a way that excluded certain classes of people on a regular basis in a way that seemed designed to deny them access to resources or personal connections which might be of interest to them, if resources were assigned to one group and not another in a way that suggested gender-based favoritism, etc.
In the situation that you described, setting up one event out of four to be gender-specific sounds reasonable to me.
You might consider talking to organizers of previous WikiWomen's lunches and/or the Wikimedia Mexico women's editathons to see if they have comments.
Writing as a male member of this list,