Where did you get that it was okay to have non-consensual sex with them? It's completely not okay in Islam. You are allowed to have sex with female slaves the same way you are allowed to have sex with a wife. It requires consent. I feel like you don't have a clear understanding of slavery in Islam or its history.
I like the fact that unlike other Muslims you admit that slavery is legal in Islam (liberal Muslims tend to deny this despite of the Quran's pro-slavery stance). Having said that though, I don't agree with your argument regarding acquiring consent from slaves before having sex. The fact that the females were enslaved in the first place betrays the fact that they lost the ability to live their lives according to their own free will and desires-- an enslaved person cannot simply walk away from their master (it's called slavery after all).
The following hadith indicates the reservations the sahaba had about having sex with married women they captured in Autas [I have pasted the hadiths as they were published, including grammatical errors]:
Abu Sa'id al-Khudri (Allah her pleased with him) reported that at the Battle of Hanain Allah's Messenger (may peace be upon him) sent an army to Autas and encountered the enemy and fought with them. Having overcome them and taken them captives, the Companions of Allah's Messenger (may peace te upon him) seemed to refrain from having intercourse with captive women because of their husbands being polytheists. Then Allah, Most High, sent down regarding that:" And women already married, except those whom your right hands possess (iv. 24)" (i. e. they were lawful for them when their 'Idda period came to an end). Sahih Muslim Book 008, Number 3432
...They took captives (women) on the day of Autas who had their husbands. They were afraid (to have sexual intercourse with them) when this verse was revealed:" And women already married except those whom you right hands posses" (iv. 24) Sahih Muslim Book 008, Number 3433
Furthermore, there's a ruling in Sahih Muslim that says not only is it legal to have sex with female slaves but if they were married, their marriage is annulled after being enslaved:
It is permissible to have intercourse with a female captive after it is established that she is not pregnant, and if she has a husband, then her marriage is annulled when she is captured Sahih Muslim Book 17, Hadith 41
Here's the verse in the Quran that authorized sexual slavery:
Also (forbidden are) women already married, except those (captives and slaves) whom your right hands possess. Thus has Allah ordained for you. All others are lawful, provided you seek (them in marriage) with Mahr (bridal money given by the husband to his wife at the time of marriage) from your property, desiring chastity, not committing illegal sexual intercourse, so with those of whom you have enjoyed sexual relations, give them their Mahr as prescribed; but if after a Mahr is prescribed, you agree mutually (to give more), there is no sin on you. Surely, Allah is Ever All-Knowing, All-Wise. Quran 4:24
And lastly here's Ibn Kathir's tafseer on the verse:
...The Ayah means, you are prohibited from marrying women who are already married... except those whom you acquire through war, for you are allowed such women after making sure they are not pregnant. Imam Ahmad recorded that Abu Sa`id Al-Khudri said, "We captured some women from the area of Awtas who were already married, and we disliked having sexual relations with them because they already had husbands. So, we asked the Prophet about this matter, and this Ayah was revealed... Ibn Kathir
All of the sources quoted-- Quran, hadith, and tafseer-- authorize sexual slavery and none of them mention anything about consent.