Earlier quoted context omitted.
Sexism[0] isn't the right word here, since it implies the belief that the victims are less able, which women are clearly not. I won't assume you hold that belief. Discrimination[1] is more neutral and is the better word to describe this situation. I do occasional technical talks or workshops for free and some organisers have asked to make them 'girls only'. I've always declined and made it clear that everyone will be…
I presume that you also refuse to do talks or workshops where all the speakers, panelists or mentors are of one gender or race? If so, good on you. Look, you sound like a nice guy, so I don't mean this as a personal attack, but your take on this is a classic example of how good people can fall into the trap of their own privilege. You don't do events where men/boys are not allowed to attend (e.g. girls who code) and…
Similarly, there would be a difference between an all-male conference to which no women were invited and an all-male conference to which no women submitted.
To state the obvious, exclusion is the differentiator, and it is not counterintuitive to be simultaneously in favor of promoting women while also being against the exclusion of non-women.
My wife founded local chapters of a women-focused Girl Develop It organization, so I dearly understand that the issue can be divisive, but I personally find it hard to judge someone on whether they choose to be exclusive or inclusive, and not every event needs to conform to one "right" moral standard against which all others will be deemed deficient.