Well duh farmer boy
"Michelle is in the wind and no one know where she's gone, you got yourself a sweet androgynous girlfriend and I hightailed it outta here on a bus to nowhere, but do you think it's too late for the three of us to be together?"
@Fyb, there's a finite amount of time in this earth, there's no point in taking part in doomed experiments just so that Kara can tell Peter 'no you don't ACTUALLY want a relationship' for a few months more.
I don't think it's necessarily doomed. Together they might have brought each other enough stability to hold together. I don't see any reason to not have at least tried it. If life's too short, it's definitely too short to miss out on any positive experience you could have, even if they're temporary.
The core problem of "Peter specifically wants something that Kara specifically says is off the table" doesn't go away just because Michelle is involved too. He didn't miss out on anything he just has a different positive experience instead.
It's just that I REMEMBER this. It wasn't him running away from things, it was her shutting it down, hard, the relationship part, the emotions part, the commitment part. This is so weird.
But who knows, maybe they'll get to that part tomorrow? :)