Imagine you're a woman. The legislature that governs your country is more than 80% male, and that's after a lot of very recent gains by women. You feel this status quo is the direct result of historical as well as current bias against women, and bias for men. Much worse, since this legislature makes the laws and performs checks on the other branches of government, the inclusion or lack thereof of women in the legislature has concrete and far reaching impacts, such as whether congress takes seriously and pursues reports of sexual harassment and discrimination in the military, or whether abortion rights are upheld. This status quo is not simply the result of age-old and deep bias, it is a player. It perpetuates the bias and slows our evolution out of it.
It is in your interests to have a government that is representative and understands your difficulties as a woman in this society, that understands these biases and is highly motivated to undo them. You would also like your daughter and the daughters of others to one day look at that poster on the classroom wall with a solid block of 44 male presidents in a row and finally see a woman among them, because you know it has an impact on their psychology and dreams and perceptions of gender, even if only one woman among 44, even if it takes decades for that poster to change enough to erase its impact. You directly experience on a day to day basis how much women are limited by society simply due to their gender. It is also obvious that despite the ever increasing number of men who are sensitive to women's issues, it is fare easier for other women to share your understanding simply because they've lived it and have been pained by it. It is not just a abstract principle of equality for them.
Is it not rational, in fact extremely smart, to have a preference toward electing women into office, when all other factors are more or less equal or less important? As a preference, a factor in your choice, not the sole decider, you would of course vote for a man sensitive to women's issues over a traditionalist women, or a man with unknown sensitivity to women's issues over a woman who was significantly less competent or more disagreeable with you on other important issues.
Is this gender discrimination? Is it gender discrimination if said preference only lasted as long as the stilted status quo existed? Would you not be a naive idealist to stick to the notion that despite the deeply ingrained sexism in society, one should not take any active action against it other than not be sexist yourself?