>>935644473 (OP)No slavery is ever justified, though I guess it depends how you define "slavery".
US soldiers can be legally shot for refusing orders. Are soldiers slaves? Almost nobody would say so. Some communists claim that we are all "wage slaves" because we will starve if we do not get a job and work for money. If this is slavery, then my answer would be "yes", a very specific form of sexual "slavery" is justified.
Women should be excluded from the workforce. They should be unable to get jobs to earn money to support themselves. Some young women may be supported by their parents forever, or somehow have enough savings to survive, but usually this won't work out. Prostitution should also be forbidden. Women should be faced with the following dilemma: they need a man to support them.
Removing women from the labor force would halve the labor supply, thus increasing wages for men so that they can support a woman. Women would be desperate for this support, but the men aren't going to give it away for free. The socially taught solution to this problem is this: Women must marry. Men therefore have leverage to make demands in exchange for marriage, especially of a sexual variety. Men would have an obligation to support their wives, and wives would have an obligation to satisfy their husband's sexual desires. Technically she isn't forced to let him fuck her, but if she won't, then he won't marry her and she will starve.
If this is slavery, then this is justified in the same way that people are forced to work or be homeless.