If you really gone through many sites, you must have came across the detailed answer to your question. Both from islamic point of view and other.
For you to understand why prophet of islam :saw: married more then four women, you should first understand that it was not for sexual purposes(as prophet Mohammad in his prime age, married one lady 15 year older then him and did not marry any other women in her life)
Prophet had all the children from this lady and their marraige lasted for 20 years(roughly).
Now the above mentioned fact make all the allegation go down the drain.
Now the question arises that what was the need for many more marraiges.
You may or may not know the fact that prophet Muhammad, After settling in madeena. Established an institute, in which he gave education to muslims about basic Beilefs, Ethics, law, and almost all subjects which were related to daily life of a man. The students were all male.
The need to educate woman about their resposibilities and rights, ethics and morality was very much the same.
Islam does not allow men to have close relationship with Na-mehram *women.
*Na-mehram=A person you can marry
As husband and wife have an intimate relationship, that they can talk about any thing without much hessitence. So Allah ordered prophet Muhammad :saw: to marry more women so that they can get first hand knowledge about him, his habits and tell other muslim women about it. so that they can get the knowledge about islam from the source.
And it is a fact that a very high percentage of information about islam and prophet came to muslims through his wives.
If you want to understand, then you are most welcome to question.
If you want to bash islam or prophet then I think it would be useless to waste my time in answering.
p.s I will reply about your question about girl slaves in my next post 
Take care