Dont forget Rouxls and his throuple with Elnina and Lanino!
The straight couples are Asgore and Toriel (divorced), Rudy and Carol (distant) and... uh, Toriel and Sans, maybe? Berdly has a crush on Susie, but he'll agree to go to the festival with Kris so I don't think he's straight either.
I'm honestly racking my brains trying to think of other straight characters. I think Catty called Asriel cute. Oh, those two dogs are straight in Snowdin in Undertale iirc. So that makes one straight couple that are actually together.
the fly that lives in my toilet

He was also called hot by Lanina and Elnino though, so we have to assume that they're into flat assed lamps.
Rouxls was on fire on chapter 3 tbh, the throuple business, the laser pointere, "I'm only here for the snacks." I hope so much he comes back in chapter 5