I've always assumed that the "experimenting" relationship was phrased intentionally vague to give room for misunderstandings between Ian and Lena—to blur the line of what's cheating or not. If that's not the case, I think that ideally could have been a bit more explicit in the chapter 9 relationship talk.
First, I also understood that wrong
But the actual conversation is quite clear:
Ian: "We're both in a similar situation, so I get where you're coming from. I feel a bit like that too..."
Ian: "So maybe we could keep experimenting... together."
Lena: "Experimenting together? And... how would that be like?"
Ian: "I'm not sure. I've never tried something like that in a relationship, but maybe we can talk it over."
Ian: "If the two of us are OK with it, I guess we could try a more... {i}open{/i} approach to things."
Lena: "That could be... interesting."
So at the very least no sex with other people before they 'talk it over'.
One more thing, It seems there will be Robert content in this chapter after all.
If Lena gets in a fight with Ian regarding her Stalkfap content she will go home alone and have the option to call Robert.
If you click on it nothing happens for now but it will be added in the Beta or Final for sure.
I didn't play through it but looked at the code and the scene is already there. It is just one reused picture from chapter 7 though.