Chyoa.com is not just the best branching erotic story site, it's the best branching story site on the web at the moment. It's certainly not against the rules, but what if I just start writing a normal fantasy story, that may or may not include some sex? Any rules against that?
There's nothing wrong with it. In fact, Non-Erotic has its own section. Just...don't expect as much traffic as the erotic stories at first. Most of the readers here come for one very specific reason.
One of my stories only has an erotic scene every, say, 5 or 6 chapters (which is well above average on this site). It still attracts attention. This is a pretty big site, and there's probably a niche for whatever you want to publish - even non-erotic writing.
As long as you use the non-erotic tag then I bet it would find an audience. The tag will help the readers know what to expect, or not expect I guess, so presumably no one would complain. My latest story doesn’t have so much as a kiss until 15 chapters and 25k words in and no one seemed to mind.
I always say the same thing, so... forgive me if I sound like a broken record. Write what you want to read. IMHO there's a place for anything that doesn't get you dragged out of your house at 2 AM. Including jokes about getting dragged out of your house at 2 AM.