Funny, but I have almost the opposite feeling (although it's been a while since I played through the current update). I feel like since Mia was the pre-existing relationship that somehow Sadie would be more willing to accept that than MC starting something new during his time with her. Personally I like Sadie more than anyone other than Mia, and so personally I think they did a great job with Sadie as a new character. I prefer Mia, but still don't want to hurt Sadie; the question for me is whether
Mia would ever be OK with being in a thruple.

Considering how she abandoned the MC for 2 years, and that she knows Sadie a bit at this point, I hope it isn't outside the realm of possibility...