There are ways of sharing the girls or keep them to yourself, you'll be able to pick an option with almost everyone (there are some cases where you just join an already existing couple). They won't "cheat" (since you're aware of everything) on you if that's what you're asking, I'd say just play it and see for yourself, you won't get NTR scenes if you don't select the options, and don't be too naughty/flirty with Anna at first since that will leave her in the slut route