However, most people keep arguing this point from a basis of stereotypes, and assume that those on the opposite side much simply except it even if they don't fit that stereotypes.
For me, as I've been taking the gender discrimination tact, no ones been able to show me where NUMBER of people is a protected status anywhere near the level of sexual orientation.
In regards to those arguing sexual orientation, their argument is that sexual orientation should be included at the same stricter levels of at least gender if not race/religion/etc, and have reasons supporting that. Those that oppose it with polygamy rarely have legitimate reasons why "Number of people" should be a protected status.
Perhaps the Polygamy argument would be useful if anyone ever presented it in a way other than "Yeah well, um, if you think that way you have to think Polygamy is okay too or your a BIGOT cause um....I don't have any reasons, but cause I say so!"