You will find a buyer on bdsmbooks.com if you can make all characters 18 or over. They will accept most things, but anyone even technically underage is right out, and given the current moral panic about child porn, I can't blame them; they have to stay in business. I think you'll find that this is a rule anywhere you go.

It's not insuperable. I had written a kidnapping/slavery novel with a 15yo hero, and at first I thought that making him almost the same age as his victim would ruin the power dynamics that made the story interesting. But with a little creative imagination I found a way to make him legally old enough but still feeling like a 15yo. They'll be publishing it in a few weeks!

If the character's youth is important because it means innocence, make the character a nun or monk, grownup but still a child in experience of life. If it means weakness, make them disabled. You can think your way round these things.

Looking again at your post, 70kw is long enough for a paper publisher like Silver Moon, who do a lot of heavy BDSM. I'd start there, but after you've fixed the age problem.