RSRH: The aforementioned gemorah is clear that toeiva marriage is a clear-cut violation of the 7 mitzvos.
Additionally it is clear, that these abominable promoters are openly, gleefully, and unashamedly engaged in the forbidden “action” in question. And THAT (the action) is what they are demanding be legitimized.
And like others above have mentioned, why are you and the toeiva marriage supporters not promoting legalized marriage between Mother and Son as well as between Brother and Sister?
Do us all a favor and read the gemorah. It is difficult to believe someone who believes in the Torah is taking your position. And it will be doubly difficult to believe you maintain that position after correctly understanding the gemorah.