Just wanted to share this for PC players.
The first time I played Dawnguard, I chose vampires' side and it kinda happened unintentionally, I didn't want it, but just happened so I just followed the story, I didn't have an earlier save and I was stuck with the decison. As in Skyrim special edition, all dlc's are built in and you can not disable them from in game menu.
However, you can disable them manually, very easily.
1-Save your game and quit.
2-Go to your game directory, (where your game is installed, obviously)
Go to Skyrim Special Edition/Data, find the file Dawnguard.esm and just rename it. (something like xxxdawnguard.esm) or however you like. Just make sure you don't lose the actual file name.
3-Start your game, load the game you just saved before changing the file, you will realize you no longer have anything related to dawnguard anymore. It worked! yay!
4-Save again and quit again.
5-Go back to your game directory and rename the dawnguard dlc properly, just the way it was originally. (Dawnguard.esm)
6-Start your game, load your last save, go to riften, talk to a guard and restart dawnguard questline from scratch.
This time make sure you choose dawnguards so you can get the awesome dwarven cross bow and a chance to get a husky as follower.
Yay!