I think of history for men as being inherently lonely and that civilizations conquer each other partially because there are surplus men domestically as a result

I think of history for men as being inherently lonely and that civilizations conquer each other partially because there are surplus men domestically as a result of polygamy. This is supported through relatively low variation in y chromosome DNA. Extra men are a problem but, as you say, people losing interest in future generations is also a problem.