is it best to use "flagellum" or "plague"?

Neither one makes any sense. What do you mean by "gay propaganda"? If this is an essay you are writing for school, it's not likely to go over very well. What possible difference does it make to your life what gay people do in their bedrooms and whether they can get married or not?

Neither is used grammatically in the examples you have given. 'Flagellum' sounds especially odd.

I don't see how flagellum gives any meaning to the sentence. It is an odd usage. Too obscure if you mean it in sense of a whip, but otherwise represents nothing inherently harmful so has no understandable intent.

Plague would be the more common and most understood of the terms.

A flagellum is a rod like structure that acts a limb for bacteria. A plague is a large scale disease outbreak. I don't know what "Gay propaganda" is unless it's just another fiction that cons use to stir up their brainless cultists.