iirc there's only one futa scene (the guards Peanut and Crystal from book 3), so I don't think that lines up with the " There's some of it in FET, but not enough to give it away." and "It only involves a few original non-canon characters". I'm willing to bet Marty likes public nudity/sex or exhibition type stuff since we see that multiple times throughout the later books with original characters. Either that or he's got a thing for slaves/prisoners/girls in stocks as we see that happen a few times in books 2, 3, & 4 (though with some canon characters too (Pema/Linn/Kuvira)).