To me it seems that coming out poly is indeed more difficult that
coming out homosexual or bisexual. I think this is partially just
because poly is less far up the steep hill of societal acceptance.
There is a school of thought that poly is less acceptable than gay
because straight monos can't conceive of themselves as gay, but they
can see themselves as poly, or worse yet, their partners wanting to
be poly. Whatever "bad" gay might be, gays don't threaten straight
relationships. Polys do. That is a big difference.