Status doesn't explain everything, it does explain a lot of discrimination.
Do you mean that status is a better explanation that in-group/out-group bias, or that status is equivalent to in-group/out-group bias?
It isn't fully equivalent. Out-group polygamous marriages are a-ok for us, one sees little lobbying on the UN level to forbid polygamous marriage. But I think Muslim immigrants in Europe and Mormon sects in the US are low status in-group members for most citizens when thinking about such issues.
Basically in-group out-group determines who has moral relevance. Status determines with who you wish to associate or disassociate.
A thought occurred to me today as I skimmed an article in a rationality forum where the subject of gay marriage cropped up; seeing as the issue has been hotly contested in various public fora and especially the courts, what about poly? After all, many if not all the arguments for gay marriage apply to poly marriage as well.
Questions for LWers who are currently in a such a relationship, or have an opinion to share:
Do polies want to marry each other or do such relationships not lend themselves to permanence above a threshold of partners? Should polies campaign for the right for a civil union anyway? what are the up and down sides of this? etc