People approach relationships in different ways. Often, we replicate what we saw growing up. Perhaps, he grew up in an environment where emotional manipulation was how people interacted with each other. Some people are also just like that in terms of personality. It doesn't necessarily mean he does not love you, it could just be that this is the only way he knows how to interact with people.
My experience is that the best way to deal with emotional games, emotional manipulation, and emotional blackmail is just to take a deep breath, distance one's self slightly, and call it out for what it is (either internally or in conversation). It is also helpful to separate the issues (again, either internally or in conversation) and say that taking a second wife is a different issue from whatever you are disagreeing about, and if he is serious about taking a second wife, it is a big decision and you should talk about it at some other time when no one is upset. Then, refuse to play the emotional games and deal with the issue at hand. I know this is easier said than done, this is just my experience!
Most likely, if you do not react emotionally to the threat of taking a second wife, he will stop making it. This is unless he is genuinely serious about taking a second wife, at which time you should still discuss it at a time when everyone is calmer.
There is also the possibility that he already has a second wife and, rather than being a brave manly man and telling you directly, he is looking for a way to make it seem like it is your fault and drop it on you. I don't think this is the case here and I certainly don't want to make you paranoid or suspicious, but I just thought I'd put it out there!