It just seems like a little bit of a slap in the face considering:
1)He's been pining for Andi up until two or three episodes ago. They've built that up A LOT.
2)Even with everything going on with Andi, they introduce Taylor, and at least you get to see a progression between the initial meeting, and something of a first date, at which point Sam wants to take things slow.
So bearing those two things in mind, even if we assume a long enough period of time has passed for them to now be a couple, it just seems really sloppy to present a scenario to the viewers in which Sam and Cady would go from just meeting each other at the end of one episode, to being a clearly defined romantic couple at the beginning of the next episode. I'm not talking about the passage of time in the context of the characters' reality. I'm talking about the narrative structure in the context of the viewing audience's reality. Yeah, it's evident from jump that the whole hook of her character is that she may or may not be the daughter of the devil, but we still need to get to know her a little bit better to understand why Sam can so quickly fall for her and forget all about Andi, and to a lesser extent, Taylor.