the prophet pbuh had 11 wives. some were proposed, some proposed to him and he accepted, some were those whose tribes were captured and so in order to build political ties with the tribes, he married with one of their women to strengthen family relations. some were slaves. when he married them, she would acquire the status of a free woman.
just like, in the old days, kings of england would marry princesses from spain or wales, to strengthen family ties between the two rulers.
so muhammad pbuh married into different tribes to strengthen political ties. his wives are not slaves. they are the mother of the believers.
in the olden days, if a woman was a slave, if she got married to a free man, she would be exalted in status and would no longer remain a slave. marriage was a way out of slavery for slave-girls.
slavery was common all across the world at that time. islam gradually freed the slaves by giving them their freedom or marrying.
u need to read the biography of the prophet pbuh. the evidence is in there. if you dont read, u look at only one angle of a cube. i keep saying its a cube. but u look at one angle and you say no no its a square. does that make sense to you?
