The new sex education curriculum in Ontario is one of the most comprehensive in Canada, attempting to address not only the simple mechanics of procreation and disease prevention, but also dealing with issues of gender identity, sexual health, relationship issues, and a wide range of sexual behaviors. It strives to be diverse and inclusive, and has the support of Institute of Catholic Education as well as secular organizations.
There are two main approaches you might take to writing an essay opposing this new curriculum. The first approach would be one based on the multicultural nature of Ontario schools. Especially in the Greater Toronto Area, many students come from tradition Hindu or Muslim families. You could argue from a thesis statement such as: "Some of the approaches to gender and sexuality in the new sex education curriculum attempt to impose liberal western values on students from diverse backgrounds."
Another less obvious but perhaps more sustainable line of argument would be pedagogical. The curriculum requires teachers to cover a vast range of complex material in a limited time, starting as early as the fourth grade. The CBC article cited below discusses these issues. You might therefore use as an alternative thesis: "Teachers are concerned that the new sex education curriculum attempts to do too much too early, and that it may be difficult to find teachers qualified to teach it."