If Lena and/or Ian had slept with other people before Ch.9, you'll get an option after forming a couple, to "maybe experiment together". However this shouldn't be confused with "we're a romantic couple but you can fuck whoever you like" but more like actual "experimenting" together, i.e. potential threesome(s) down the road. Making that choice isn't strictly necessary, though, Lena can choose to be interested in this sort of arrangement in Ch.11 even without it.
Also, if you are aiming for Lena/Ian/Holly specifically, then i suspect you won't need "open relationship" for it, but it might become an option if Lena goes for the heart icon options with Holly while in relationship with Ian.
It's not just in AVNs. It's fairly common attitude in society too, because dudes tend to go way more ballistic when there's another dick in their girlfriend involved.