How open do you want the relationship? Threesomes and perhaps bigger groups? Or fully going off with anyone each partner fancies?
For the first, there's an 'open together' type path both at the park and the early part of the beach-house. 'Full open' requires Lena or Ian to cheat on the other, know about it, then call the other out. There's a lot of nitty-gritty in the walkthrough, but that's the gist, then the right one needs to propose the true/full open relationship. It hurts their connection, but then they are both allowed to fuck anyone (it seems).