The subject came up in the past, a what-if scenario of a committed romantic interest sleeping with someone else while still together with the PC.
People loved the idea. Some, not so much. hahaha
Well, Jacob did that. It did not go over well. (Although there were lots of circumstances surrounding it that made it way more problematic.)
I mean, they kinda had that in Origins, right? Alistair found out if you killed Connor or Isolde regardless of if he was in your party, and Leliana finds out about the Sacred Ashes, too.
All they need to do is make it so that it breaks the romance as well.
Wynne always hears about the Ashes and leaves (even if she joins after it's done), and I think Zevran hears about it if you slaughter the Dalish. I know he confronts you about it, but I don't know if that happens no matter what.
Also, in DA2, if you take Anders into the Fade and hand Feynriel over to Torpor, he'll break up with you. Alas, if you haven't triggered a romance with him I think you can still get together afterward, though.
I'd love it if your companions called you out on things you did, even if they weren't there. Maybe you can smooth things over enough to make them stay, but a romance might be out of the question.