I was surprised to find that the national government is involved in local education, since the states generally have considerably more autonomy than states and provinces in other countries. But sure enough,
The government also held powers to make national policies for educational development and could regulate selected aspects of education throughout India.
Apparently this is one of those selected aspects.
In answer to your questions, I think that sex education can obviously be done in a way that promotes promiscuity, but it can also be done in a more intelligent way that does not.
Any other subject distracts from the 3 R’s: Phys ed, music, science, social studies, health, art, safety (don’t talk to strangers!), computers, etc. Some more than others. That in itself isn’t much of a reason not to do these subjects.