From what I've read about this phenom, this is more of a cultural/religious/tribal event that carries more of a symbolic meaning than traditional marriage does.
In cases of this, it seems that the person is usually poor and suffering from some illness which the animal (who is revered in the culture) is supposed to cure. The person can go on to marry an actual human later and not have to go through a regular divorce or anything...

So, while we think it weird, it's apparently not to this culture...