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.So they have to keep it non-romantic? I figured just having multiple hearts for each character would have done it. Dang.
*Edit: And I suppose I should have phrased it as something like 'the beginning of a romantic open relationship,' but that would be me projecting a bit. First playthrough all I was going for was a Ian/Lena/Holly throuple, but that blows up in your face hard. This could never be a harem game, but I could see them seeing each other as primaries, and fucking around on the side.
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.That is one of the most hypocritical, but exceedingly prevalent cliches in AVNs.