The law did not ban polyamory, it is already illegal in nepal, so there is no "cover of marriage" there. It banned posting pro poly posts online. That is pretty harsh. Just because criminals use it does not mean stating your opinion had to be banned!

The only explanation I can come up with is that they just banned a bunch of stuff the legislators deemed immoral. I dont know enough about nepal to say that with any certainty, but my hunch is that pro-poly messaging may have been banned because of the muslim minority.