Im curious as to why in the past men slave owners could have relations with there slave? What was the reasoning behind that? Also I understand that slaves were captured as a result of them waging war on Muslims but why were women also captured too? Did these women slaves actively fight against the Muslims too because I thought it was usually men who would go to battle?

You may see this video at 17 minutes until the end to answer your questions:

https://youtu.be/EUHIK5r6IXk