I've read a lot of romance stories, and I'm working on four now. But I notice serious hate for "nontraditional" couples (meaning monogamous or popular). I personally like rare couples, but since they get the tag "crack pairing" it's automatically looked down upon. So my question is, what (if any) is the extent of a consensual romantic relationships?

Well, it seems like most people go with what they like and frown upon everything else. Basically, if you don`t like Trixie then you will hate any pairing including her. The most popular shippings are TwiDash, Twilestia and AppleDash. If you don`t write those stories then you will get dislikes simply because they don`t enjoy the pairing. I personally like crack ships and they even have a group dedicated to them. I suggest joining that group and putting all of those stories there. You will get crack ship fans on your stories instead of a bunch of haters. I hope that this helps!