No babe, the ML is Maximus and he is a freaking green flag ever. He did not keep or sexually anyone, he had sex with so many pretty boys before indeed but just normal sex. Most of the boys used to had sex with him even fall in love for him lol, so don't worry, he's the one healed the MC after all tragic stories he suffered!
No, it's not even love triangle because Tiberius is the reason of all the tragic stories the MC had suffered, the only emotion left of the mc to Tiberius is fear, I just read half of the novel so that's all I know, but I'm sure it's not 3P. The way Tiberius kneel and begging Hadi for forgiveness is disgusted me, rot in hell!!!
A bit late but to actually address your question, the uke is sold into sexual slavery yet the way it works is that the slaver is his pimp. He is the one who owns him.
The slaver then sells his slaves like prostitutes for sexual encounters.
The blonde endgame seme basically just goes to brothels for sex and gets these "sex slaves"for momentary pleasure and then sends them back or leaves.
He does not own a sex slave. It's more like he uses prostitutes/sex workers (as it seems more consensual in the brothels).
He does have normal slaves though but that's basically indentured servitude and just standard reality for the Patrician class of this time period
If it matters, I'm the biggest hater of semes and I found him to be an exceptionally green flag for these kinds of stories.

I genuinely don’t care if you spoil everything for me because I’m specifically asking for spoilers.
I heard that the MC was sold to the ML as a sex slave, and that at the beginning the ML treated him like he was just another disposable replacement that he could use and throw away.
What I specifically want to know is: before the MC came into his life, did the ML also buy, keep, or sexually use other slaves? Did he have other people who were essentially sex slaves/sexual servants under him?
Because this is a major boundary for me. I don’t care if the story is set in an ancient or historical period where slavery was normalized. If the ML himself participated in sexually exploiting slaves before the MC, I personally cannot overlook that just because he eventually becomes a “green flag” for the MC.
So please spoil me completely: Was the MC actually the first person the ML ever sexually used in that way, or had he done the same thing with other slaves before him?