Isn't there slavery (sexual and otherwise) everywhere islam went ? I mean, the prophet kept slaves (and had extramarital sex with some of them, and this is not doubted or controversial). He also forced some of his other slaves to work, and he confirmed the right for an owner to do whatever he wanted with his/her slaves, including controversially kill them without the need for an explanation.

Why would it be any different in the Ottoman empire ?

First of all, I did not say slavery did not exist in the Empire. It did. But the Janissary/Devsirme is not it. IDK much facts with their proofs about slavery in the Ottoman Empire or Islam, but I doubt the truth of your statements. I'm not here to defend the religion or the empire, but I have never heard of those things (except the liberated Zayd), and I'd be reluctant to take religious myths and gossip as literal facts, especially if there are doubts the people we're talking about ever lived. Then, even if we accept your affirmations, there's no reason to believe that it inevitably causes the Empire to behave in a certain way. Moreover, slavery existed in almost all parts of the world and for the most of our common past, and even made its way into the 20th century in some. So tieing it to a religion is not really logical. I think of religion as a consolidation of a people's tradition, and looking at Ancient Greek and Ancient Mesopotamian literature, it's not hard to find where slavery comes from in the Eastern Mediterranean.

Returning to the original topic, the devsirme is nominally slavery, but the substance of the phaenomenon is not really. And some sultans practiced paderasty, but again reducing the devsirme to a periodical collection of homosexual male sex slaves is just not correct. We're talking about the most powerful men in the Ottoman society for the entire existence of the Janissary/Devsirme system. On many occasions they decided and/or dethroned sultans. Many devsirme dynasties obtained immense power and wealth. The Sokollu family ruled the empire for decades and decades. Mehmed Sokollu created the Bulgarian Church and made his brother the patriarch. Until Mahmud II, not bribing the Janissaries enough meant death for the sultans. Now don't tell me these men were succumbed into a system of involuntary rape by that one guy which they get rid of when they want anyways.