only marriages between a man and a woman shall be under the protection of the Polish state.
for the same reason that doesn't state that marriage between a man and a dog shall be under the protection of the state or between a women and her son or between a man and a child. Based on an obvious assumption that when one talk about marriage its pertain exclusively about act between a man and a woman.
Now you can introduce any HS and call it law.
religion lessons are pretty much universally loathed by high school students in Poland.
Which is pretty meaningless as long as their parents think that it is the best thing since sliced bread.