Out of curiosity, why is it so important to us that candidates are heterosexual and are against same gender marriage? Is it really our responsibility to make sure that Goyim keep halachah? Even if you assume that this is one of the 7 Mitzvos Bnei Noach, why should it bother us? It doesn’t bother us when a candidate is Christian or promotes Christian values even though Christianity is probably avodah zarah and also one of the 7 Mitzvos Bnei Noach (assuming that shituf is Avodah Zarah for Goyim).
Maybe these Rabbonim hold that it is not our responsibility to make sure that the Goyim are shomrei halachah… but rather our responsibility is to our own community and that there is ample funding from yeshivos and other Torah institutions. Why should we vote for a candidate who may mirror are values more accurately, but will be less beneficial for our yeshivos? Let’s look out for ourselves first.