You are dealing with a poster who would make George Orwell proud. I think his intent was to refer to advocates of gay marriage. He has twisted that to monogamy and used it with a bad connotation. I think if more gays were monogamous that would be a good thing, would it not?
Yes it would.
That would make them heterosexual. The word monogamy denotes a biological procreation as does polygamy... homosexuals can do neither of them, their union does not produce offspring.
Homosexual activity is never a good thing.