Advocates of retaining the millennia-old definition of marriage as by nature heterosexual are on the defensive everywhere, and it doesn’t always help that, for many, their principal reason for resisting the current juggernaut for redefinition rests on either the “eeew” or “ick” factor or on some variant of “because the Bible [or the Prophet] says so.”
Such reasons easily dismissed by triumphalist secular progressives as irrational.
This site might be helpful:
Thanks to Jeffrey Thayne for bringing it to my notice.