This question has been asked to here, it's been asked to other Polygamy friendly families as well. Why are there usually more women added to the family and not other males? M wrote about this in a comment on her blog and discussed the difference between polygyny and polyandry. She also discussed that polygamy or polygyny is Biblical, while polyandry is not. I'm not going to address those points, as I think she did a pretty good job. Instead, I'm going to share with you some other thoughts about the subject. Recently, I watched a TV documentary that touched on this. It was not addressing Polygamy directly, but societies and the animal kingdom with multiple partners.
The program had a real simple explanation as to why females tend to be the majority in plural relationships and why there is usually only one male. Females tend to be more social and accepting of one another in groups. They tend to be motherly and directly involved in caring for one another's children as if they were their own. They also said in general women/females are more open to sharing their possessions once they have a bond with another female. There is a special bond that women often get once in close friendship with another women which makes them more open to living in a plural setting. I would say that that is one reason why women are more open to having sister-wives, not all women, but some women. Perhaps, one might even go as so far as to say that is why scientists have found that female brains are very different than males.
On the other hand, the program put it pretty plainly that males are selfish and do not share. At least when compared to females, even in the animal kingdom. We covet our possessions and deep down primal males consider females possessions. I do NOT look at my wife as a possession, but I do feel that we are one, united and truly part of one another. The program used many different situations showing how males protected what was perceived as 'theirs'. The showed how in the animal kingdom males fight and kill to protect their female/females. They used examples from long ago of how fights, duels and battles were started over ones wife/wives.
If you think about it from a Biblical standpoint, it is a sin to covet another man's wife/wives. This reinforces feeling united with your wife/wives. Which reinforces those protective feelings, making it much harder to even consider sharing your wife/wives with another man.
This information was just from something that I watched the other night. I thought it was interesting since this question is often brought up on other blogs and forums. If you take the time to look at it, you can see it. I can see it......My wife and I go out from time to time with friends which usually happen to be girls. When we go out for a night of live music and dancing. She has no problem with me dancing with the girls and it is all just clean fun. But I will tell you this from a males prospective.....when I see a guy moving in and trying to get into the group........My spidey senses get into a lather!!! The primal thoughts of "these girls are here with me" and they are invading starts up....I know I lay no claim to any of them except my own wife. But when you start out the night as the only guy getting the attention and another tries to edge into your world......it is not good. Often these guys try to make their way in and before you know it they are causing problems. More than once I have had to ask a guy to step back.... I'm protective, I will admit it. It's actually one of the reasons why M's girlfriends like to go out with us. It's also another reason why some of them who are married or in relationships, prefer to go out with us if their guys aren't able to take them out.
So, I will close this up by saying this... Multiple men sharing one wife doesn't work in most instances. Excuse me for saying so, but we are often too damn jealous, too possessive, too dominant and protective. We are selfish by nature and there would be nothing but trouble. I mean lets not even start with the procreation battles that could ensue, after all in polygamy if two women want to get pregnant, no problem. But if two men want to Father children at the same time.......jealously over who gets to be the next Father, who has the better genes, who is the better Dad......Oh man....that would start a war from within......those are just my thoughts.
I know others will have a different point of view, feelings and or experiences. But this is my blog and I share what information I come across along with my views and feelings.