This is a question that often comes up in objection to polygamy - aren't you enough for your husband? And the short version of my answer would be - Yes I am! That is why we want more... But in order to explain this statement, I believe I shall have to elaborate a bit. As I have done in others of my articles I will use a parable that is easier to relate to for those of you who happen to not be polygamously married.
Let us ask the same question in a different context - why have another child, isn't your first enough? I myself happen to have one 3 year-old child, I am very content with him, and as I am his mother I would have great difficulties trying to describe his faults! My love for him simply blinds me. But that does not prevent me from wanting more children, sisters and brothers for my child. All of them will be just as precious to me.
Now, wives are not children. Marriage is more binding than a parent-and-child relationship, and the love has an extra dimension, but I think the principle is the same. A child who is not certain that his parents still love him will resent his little sister or brother, whereas one who knows that he can never be replaced will open his arms freely.
I think all people - not only children - need to know that they are loved: First and foremost by the never-changing Elohim (God) who will always fulfill our needs as long as we let Him; And next by our immediate family. And like the child who trusts it's parents' continuing love, a wife who is confident that her husband's love for her will not diminish, will not feel threatend by a new wife, but welcome her and try to give her the best possible start in her new family. That is at least how it works in my life.
Of course, like children, we are only humans and may have conflicting interests at times - but this is where we as grown-ups have an advantage. We don't hit each other on the head or tear our husband apart between us - we realise that since we are going to spend our lives (also the comming one) together (children part when they leave home) we talk things through and try to understand or at least accept each other.
I am a Christian/Messianic, and I rest secure that Yah'shua (Jesus) loves me more than anyone else can. Therefore I need not fear. If another person converts to Christ I am thrilled, not worried that they might occupy my place in the Saviour's heart. I know He is still just as pleased with me even if another pleases Him as well.
Similarly, I have no doubt of Stanisław's love for me, and I know that he is just as satisfied with me as I am with him. My sister-wives are not competitors or dangerous ... they are my lovely companions - is it any wonder that I want more?