Nah, it's more that if it's in the Sex category, sex is what needs to be happening. More often than not, like in the attached picture, sex is not what's happening here. In this picture, she's showing off her tits and he's jacking off. Not sex.
Ecchi category, I see a lot of nude pics, and I don't think they're really appropriate for the Ecchi category in this game. The pictures in this category should definitely be safely in the Not Safe For Work category, but more for how the girl is acting than what she's wearing. The girl can be wearing a burka, but if she's in the process of stripping it off and the important bits are starting to show, it would go in the Ecchi category. On the other side of things, most pictures where she is completely naked don't really fit, especially bathhouse scenes. She's undressed, but she's not acting ecchi, if that makes sense. Or maybe I'm just biased because I get off more on partially dressed girls than fully undressed.
Another point against the nude pics is that the pics in the Ecchi category will be shown in the Sleazy bar or any stripping category jobs. In the stripper job, she's being paid to undress. In the sleazy bar, the barmaids and waitresses will be serving the customers (but not 'servicing') while wearing fetish outfits or lingerie.