Just don't mention anything about how old Maria was at Omeros' birth. She's 36 now. All good.
I understand that many men might find castration distressing
, and some might find reading about it to be a bit uncomfortable, but it's certainly not shied away from in slavery images. There are examples of artwork on CF depicting the actual act of castration. If this is the case, how can there be any reasonable objection to depicting castration in a story? I doubt much of the audience will be lost.
Also, if he is castrated and kept as a Harem slave, why would that be an uncrossable line? Just last week, I saw member artwork depicting a man who had been castrated. His wife was now the sex slave of the man who bought the couple and he was a house slave in the same place, often having to serve (or so the picture implied) his master while his master used his (former) wife.
There are, of course, uncrossable lines, even at CF (no real crux with nails. no real snuff videos/images, no glorification of real historical atrocities, etc.), but slave castration in fiction is not one of those lines.
Anyway, if Omeros is going to cause problems, you could write him out (sell him to another owner, send him off to the master's fields, never to be seen again). That way Maria can at least imagine he's alive somewhere and not mourn forever, and you don't have revenge problems.
If he is critical to the plot, and he has to be in the harem, I suppose he'll just have to suffer the consequences.