Can I try and answer this as a non-muslim who has often had this conversation with muslims? Also, as a bit of a feminist, some of these issues are quite dear to me. Here goes...
Equality does not mean that they are the same in every single aspect.
I always thought that they are equal, but in different ways. Men and women can (generally) do the same things, but in some cases, one gender can be better suited to carry out certain actions than the other. We are created equally, but complimentary to one another, as a team, we work better. It's a hard concept to explain. For example, even though women can train very hard to gain physical strength, men will be physically stronger if they put in the same effort. This doesn't mean some women aren't stronger than some men, but men and women's bodies are built differently.
As for the wife question, I have often wondered myself. There may be a few reasons. Here are some I have come up with:
Often men are the main breadwinners in the family (due to biological reasons: women cannot usually work for the entirety of their pregnancy and time to recover is needed. This can often affect you job/career and money coming in). So for a woman to have 1,2,3, etc husbands, it would put a lot of stress on her both biologically (constant pregnancies), socially (take care of children) and professionally (if she is often pregnant, then there is no time for work and money making). Men don't have to take time away from work when their family expands.
More along the line of biology, the survival of the species depends on procreation. The easiest way to procreate is to have a higher female:male ratio. It's basic biology and reproduction.
More on the social side: again, back to the breadwinner theory: Even though in many societies, women can and do work, in other societies it is not as readily accepted (now as was in the past). It can sometimes be very difficult for a woman to survive on her own if she is single, and even more if she is widowed. The situation in some countries is so bad that women have to resort to prostitution just to be able to feed their children. Marriage can be an option to alleviate this situation.
I hope I made some sense.