I agree with this law. Peddling this crap to minors should be considered a form of child molestation.
I think the main idea behind this law is responding to a public outcry over gaypride rallies.
In Moscow there is a practice to view it as delinquent on a legal basis of disrupting traffic, public nudity display etc to charge participants with minor charges after.
St Petersburg is traditionally more liberal so does it’s judges. This is a law to make things straight for them and make no room to drop charges ruling disrupting traffic as a freedom to assembly and public nudity as freedom of expression.
It still pretty liberal in form as nobody gets jailed but it is unlikely for a rally to happend if every participant will be ticketed by every cop at $100 as obvious propagandist numerous time a day.