Way offtopic, but wouldn't fairness and common decency say that in a society with nearly 50% male and 50% female, everyone should have one spouse. But in a society where 75% of the men are killed in battle, that a 1:3 or 1:4 ratio starts to make sense?
It doesn't just have to be one man + several women. Could be one woman with several men, several men with several women etc. Also it doesn't have to be "forever" but temporary. Read Heinlein's "Time enough for Love" for a good elaboration on how such a society might work and why it might be desirable.
I'll just leave one thought here and stop hijacking this thread:
Right now many people think because the male/female ratio is approximately 50/50, that most people can get a partner. And once they get them, many succumb to the illusion that now they "have" them for the rest of their lives and often try to seal this illusion with silly things like marriage. This in turn gives them license to feel like they are being owed a partner by society which often results in a their lack of effort to actually try and be a good and desirable partner. In a society where romantic relationships are multiamorous a temporary the threat of your partner leaving you or yourself not being even able to get a partner at all will presumably put enormous pressure on people to actually try and be partners worth having. And let me tell you it would be hard to convince me that that's a bad thing
Most monogamous relationships seem to be based on fear, insecurity and ego-driven possessiveness, whereas I'm personally much more interested in love and freedom. But I do acknowledge that my views on this matter are even more unpopular than my views on the state & the government so I'll just leave it at that 