Opponents of sexual education in schools say the programme would encourage "gender ideology", in reference to elements of the sexual education curriculum that focus on homosexuality and transgenderism.
"Argentina's education system is chaotic and on top of this they want to push sexuality for our children. We don't want them to encourage ideology and indoctrinate our children", Néstor Mercado, an organiser of the event, told the TN news channel.
Mercado said he respected "the decision of a homosexual person or a lesbian lady because it's their life", but added "there are biological laws that say that one is born a man or a woman".
"We are opposed to them imposing on us, the majority, a model for a minority, which represents 0.14 percent (of the population). Why do our children have to learn this way of behaving?", he said.
For his part, Alejandro Geyer, the coordinator general of the anti-abortion group Marcha por la Vida (March for Life), hypothesised that since feminist groups "lost on August 8" they now want schools "to teach how to carry out abortions".
"The same women who want sexual education are the ones who support legitimising abortion, which is the murder of children", he said.
In September, Argentina's Lower House approved a draft ESI reform bill at committee level, which would restrict educational institutions from picking and choosing aspects of the sexual education curriculum, instead requiring full compliance.