Like already suggested, it’s historical an cultural.
I mean, your dad and you may be fine with polygamy but would you be as cool with, say, polyandry (mutliple husbands for a single woman)? If not, why? And what about monogamy or even not being married and practicing ‘free’ sex (partners mating and then splitting freely, willingly)? Or celibacy edit: chastity/abstinence of sex?
Those are all cultural/historical/societal values. Some of those values are closer to our own personal values. Others are definitely not. Some are at the complete opposite of the spectrum of one another. But they’re still all based on principles, values, and on traditions that each group, if not all, will dearly defend and argue is the better choice if not the only ‘true’ one. I’m thinking hard but I can’t recall any noticeable group that welcomed ‘alien’ sexual/marital practice.
Heck, even our good old own hippies of the 60s and 70s, with their ‘free’ or liberated sex and love were still openly hostile and quite dismissive to the traditional ‘married couple’.
All questions related to sexuality/relationships and attribution of power (things like who is head of the family, who should be allowed to get specific kind of jobs (say, be a priest), the age of consent and the gender of partners, the type of sexual practice that are frowned upon, the (un)willingness to have sex, and so on) are among those core values that hardly any group of population is willing to discuss. At least not their own values because, based on what I can see, most of them seem to be more than willing to openly question any other group’s values.
True that, but those persons still lack any legal status, unlike married or 'pacs’ed couples. I mean legally, in such a, err, cluster of person would be considered the head of the family and no one would be able to take medical decision for the others if they needed (they aren’t considered ‘family’) and no could inherit from the other(s) I mean not tax-free like in a standard couple. And the moment they’ve kids… Things can become real hairy. Edit: well, exactly like you mentioned already (I should have read the entire comment :p) but the point is that they would lack any legal status (like up until recently homosexual couples up until recently would have lacked one as they could not get married or pacsed).
At the very least, feminists (young as well as old ones) would frown upon that. And I think most people would look down on it, even if they may not be openly hostile.