Phil Lord’s original script for Into the Spider-Verse reportedly included a bit of their romantic storyline, but that was ultimately cut in service of taking time to introduce all the characters and to avoid overstuffing the story.
A romantic storyline between Miles Morales and Spider-Gwen is canon in the comics in a 2017 issue titled “Sittin’ in a Tree,” so the decision to implement it into the animated sequel isn’t too much of a surprise to fans.