So I know this isn't necessarily the point in these types of games, but I usually like to play through multiple times being pretty much monogamous, so it was pretty jarring when I suddenly fucked my trainer in the hot tub. I'd never done any flirting with him, hadn't peeked, hadn't even agreed to the full training package.
That might be a bug. There were definitely options that imply that you don't do anything with him, and you're definitely supposed to be able to pick a monogamous route. I didn't pick those options, so I'm not sure what happens in that case, but it's unlikely to be intentional that you can't avoid it.