One thing to note is that with Islam and the Quran, even amongst native arab speakers there are variations in the translations of a verse. This has to do with several things, not the least of which is that arabic now is different than arabic then. Another very important mitigating factor in the translation and understanding of a verse in the Quran is the circumstance in which it was revealed.
In the case of the verse you're referring to, about marrying slave women Quran, chapter 4, verse 24... another translation can be found that says :
[4:24] Also prohibited are the women who are already married, unless they flee their disbelieving husbands who are at war with you. These are GOD's commandments to you. All other categories are permitted for you in marriage, so long as you pay them their due dowries. You shall maintain your morality, by not committing adultery. Thus, whoever you like among them, you shall pay them the dowry decreed for them. You commit no error by mutually agreeing to any adjustments to the dowry. GOD is Omniscient, Most Wise.
In this case, it appears this verse was revealed as a result of a woman who captured as a slave during war, converts to Islam and wants to marry again but her husband, being a alive and well in enemy lands for obvious reasons can't or wont grant her a divorce. Furthermore, by marrying the woman she's gained the rights and respect that a wife has. And to marry her you need permission as such, from her and her guardians. Any child she bears while married is also free.
The same wikipedia article you linked notes:
Fornication (sexual intercourse between a man and a woman who are not married to each other) is regarded as an offence. Muhammad gave exemplary punishment to owners of brothels that were operated using their slave-women for such pleasures.
Of course a backwards Muslim looking for justification for a quick screw or something much worse will look to this verse and other translations and find what he's looking for, just as a person looking to vilify Islam, the Quran and Mohammad will do the same. That's the problem, a limited interpretation. The same ideology that allowed for limited interpretations that have prevented Christians from getting divorces, contraceptives, allowed for the crusades and the inquisition, prohibited a proper burial if they committed suicide and many other stupid backwards prohibitions. It took Christianity nearly two millenia to get to where it is now, and it still has a internal strife. Jewdaism has its own many conservative laws which most would look upon as archaic (or even Islamic?) as seen in their ultra-orthodox communities and they've had considerably longer to come to grips with a changing world and the changing nature of human rights.
Give muslims the benefit of another 600 years and I'm sure they'll come around?
Just reiterating my original point, it was a framework from which muslims became better people, and for a great long while were considerably more civilized and cultured than the west, but a neoconservative movement picked up at some point (read http://en.wikipedia.org/wiki/Muhammad_ibn_Abd-al-Wahhab ) and retarded the natural advancement of human rights. Part of it was in response to a rising colonial power to the west, part of it was due to divisive domestic politics... If anything the US and the modern civilized world should consider what happened to the great Islamic empire as a warning of what can happen when progress is shunned and fundamentalism is exalted.