“If a man has sexual relations with a woman who is a slave, designated for another man but not ransomed or given her freedom, an inquiry shall be held. They shall not be put to death, since she has not been freed, 21 but he shall bring a guilt offering for himself to the Lord, at the entrance of the tent of meeting, a ram as guilt offering." Leviticus 19:20-22
“When you buy a male Hebrew slave, he shall serve six years, but in the seventh he shall go out a free person, without debt. 3 If he comes in single, he shall go out single; if he comes in married, then his wife shall go out with him. 4 If his master gives him a wife and she bears him sons or daughters, the wife and her children shall be her master’s, and he shall go out alone. 5 But if the slave declares, ‘I love my master, my wife, and my children; I will not go out a free person,’ 6 then his master shall bring him before God. He shall be brought to the door or the doorpost, and his master shall pierce his ear with an awl, and he shall serve him for life." Exodus 21:2-11
"Urge slaves to be submissive to their masters in everything, to be pleasing, not talking back, 10 not stealing, but showing complete and perfect fidelity, so that in everything they may be an ornament to the teaching of God our Savior." Titus 2:9-10
“When a slaveowner strikes a male or female slave with a rod and the slave dies immediately, the owner shall be punished. 21 But if the slave survives a day or two, there is no punishment, for the slave is the owner’s property." Exodus 21:20-21
"The Lord has greatly blessed my master, and he has become wealthy; he has given him flocks and herds, silver and gold, male and female slaves, camels and donkeys." Genesis 24:35
"And the nations will take them and bring them to their place, and the house of Israel will possess the nationsa] as male and female slaves in the Lord’s land; they will take captive those who were their captors and rule over those who oppressed them." Isaiah 14:2
"When you go forth to battle against your enemies... and you see among the captives a beautiful woman, and you desire her, and would take her to you as wife... after that you may go in to her, and be her husband, and she shall be your wife." Deuteronomy 21:10-14
I mean, it's pretty clear. That's immoral behaviour. Hugely immoral behaviour. And God endorsed it.