However, two of the principal organisations representing teachers, the Federation of Religious Spanish Teaching (FERE) and the Confederation of Spanish Teaching Centres (CECE), have rejected the plan, the latter saying it will produce its own textbooks "in a fight without quarter".
The debate over the Organic Education Law is set to cause as much uproar as the approval of homosexual marriages last year in Spain. Due to come into force in 2007-8, the law introduces a requirement to teach "Education for the Citizen". The new subject, put on an equal footing with maths and language, incorporates a controversial strand on the "transmission of moral values" that will include teaching about "the family in its distinct forms".
Luis Tomas, a member of the education commission which drafted the proposal, said: "Among the students there are homosexuals, and the system has to give an educational response. It would be cruel to exclude it."
A FERE representative said: "You start by acknowledging that homosexuals can form ordinary families and you end by accepting that a family can be formed by one man and three wives."
A spokesman for CECE said: "In religious studies we will teach one set of values and in education for the citizen it is proposed to teach another. We will print our own textbooks."
The absence of control over the publication of textbooks for particular subjects appears to make this a realistic possibility. The overriding rule is that the textbooks "fulfil the spirit" of the Organic Education Law.
Historically in Spain the Catholic Church has been exclusively responsible for the teaching of values in state and private schools, which was compulsory under General Franco's rule. Since 1985 pupils have been been able to choose between classes on "confessional religion" or ethics.