That's how certain things work on the store, unfortunately. If the app's content isn't inherently sexual, and as you said is through DMs and stuff, the NSFW can get bypassed. Except for dating apps, choose your story games and AI chat apps bypass the ban as well. Sakura, the AI chat service had porn on it until a few weeks ago, now they ended up blurring it, but only in the overview, you can still see total nudity if you open a bot. Choose Your Story apps don't have nude pictures, but the text does often contain very explicit content, like heavily detailed sex scenes, and large amounts of curse words, and somehow they still manage to get a rating of PEGI 16 etc.

Unfortunately for this game, even from a single photo, it's obviously inherently sexual, so neither the App Store nor Google Play will allow it.