Did Muhammad(pbuh) force sex upon his female slaves?
The Prophet Muhammad (pbuh) never had a slave. So, enough said.
Rather, the prophet Muhammad(pbuh) bought slaves and set them free and/or adopted them. Zayd Ibn Haritha was a slave. The prophet(pbuh) bought him from the market place to set him free, then the Prophet adopted him as his son and called him after his name (that was before adoption was declared forbidden in Islam), but later on called him after his father.
Regarding Safiyyah, The Jewish Wife of Muhammad
It's amazing how far the islamophobes and anti-Islamic websites, will actually go, to lie about Islam, Allah, and his prophet Muhammad(pbuh).
Muhammad(pbuh) did NOT force himself on a captured woman on the same day that he killed her father, husband and many of her relatives. Far from it.
We find, instead the following:
1) Indeed, when Bilal ibn Rabah, a companion of the Prophet, brought Safiyyah along with another Jewess before the prophet Muhammad(pbuh), by passing through the Jews that were slain in the battle, Muhammad(pbuh) reprimanded Bilal and said "Have you no compassion, Bilal, when you brought two women past their dead husbands?"
2) The prophet (pbuh) paid for her freedom
3) It was a companion of the prophet (pbuh) who suggested that since she was a daughter of Huyayy ibn Akhtab, the leader of the tribe of Banu al-Nadir, hence only the Prophet(pbuh) was fit enough to marry her. This significant act of marrying Safiyyah(R) was indeed a great honour for her, for this not only preserved her dignity, it also prevented her from becoming a slave.
4) The prophet muhammad(pbuh) told Safiyyah that he offered her the choice between remaining a Jewess and returning to her people or entering Islam and becoming his wife. 'I chose God and his Messenger', she said. Shortly after that, they were married.
5) Safiyyah knew Muhammad (pbuh) was a true prophet from her own father, but her father rejected the prophet (pbuh) because he wasn't a jew.
6) Safiyyah also wanted to marry the prophet Muhammad(pbuh) because of a dream she had. Three days before the Prophet (pbuh) arrived at Khaybar, and Safiyyah fell captive in the
battle, she had seen in a dream a brilliant moon coming out from Madina,
moving towards Khaybar, and falling into her lap. She later said: 'When I
was captured I began to hope that my dream would come true.'
7) When she became a Muslim, the prophet muhammad (pbuh) married her, she did not change her father's or her grandfather's name even though everyone knew that they were non-believers, and that they were the enemies of Allah and his prophet (pbuh).
8) The prophet Muhammad(pbuh) allowed her to visit her family, even though her family were the enemies of Islam, and hated the prophet(pbuh).
9) The other wives of the Prophet(pbuh) used to show their jealousy of her by making slights upon her Jewish origin. But the Prophet(pbuh) always defended her. Once Safiyyah was vexed to the extreme by the taunts of all the Arab wives of the Prophet(pbuh). She took the complaint to the Prophet(pbuh), who felt great compassion for her. He consoled and encouraged her. He equipped her with logic by saying: "Safiyyah, take courage and be bold. They are in no way superior to you. Tell them: I am a daughter of the Prophet Harun (Aaron), a niece of the Prophet Musa (Moses), and a wife of the Prophet Muhammad". This is thus an excellent example of the Prophet Muhammad(pbuh) trying to wipe out pre-Islamic anti-Semitism amongst the Arabs.