There wasn't anyone saying that Rose's relationship with Scorpius was forbidden. Nobody could really find much about his personality to hate him, really. His manners were impeccable and he was the only guy she knew her age that apologized when he burped. The fact that he maintained eye contact in a conversation for longer than a few seconds made Rose's heart melt. He cared about what Rose had to say, no matter how inane or random.
Scorpius wished everyone a good rest of their day at random points in the day, which put smiles on the faces of his peers. He meant the greeting with the bottom of his heart, and his sincerity won many people over. Girls were jealous that Rose dated Scorpius, half because they were genuinely in love with him and half because they wanted a boy who was a fraction as sweet and kind and polite and wonderful as Scorpius.
He could be called clingy or a pushover, but those were faults people tended to gloss over in favor of singing his praises. Even Hugo, the most judgmental brother a girl could have, seemed to enjoy his company. Or at least, not loathe looking him straight in his ocean-blue eyes when Scorpius spoke. Any kind of approval from her brother meant more to Rose than she could ever articulate.
Aside from his intimidating parentage, with his father being a reformed Death Eater-in-training or something similar to that and his mother's ambiguous stance on He-Who-Must-Not-Be-Named who vehemently denied anything about her beliefs when asked, he was basically the perfect boyfriend.
"Why do I find myself wishing our relationship was forbidden?" Rose asked, not expecting an answer from anyone. She found herself not paying much attention to the book she was reading, or the fact that the library, where she sat alone, was full of people whispering students, studying and not noticing her.
It certainly would spice up our relationship, Rose thought, her thoughts too bitter for her own liking. He was too good to her, and that was a blessing Rose was mostly thankful for. But at times like this, she wished their relationship could be half as tumultuous but fascinating like the ones she read in books or Witch Weekly. She wanted a relationship where people questioned why they were still together and furiously followed their every move with a sense of morbid fascination.
After a few seconds of silence to fantasize about what would happen if her relationship problems were exaggerated so much as to transform into something only acceptable in fiction, Rose stumbled upon a simple but effective revelation: "If I changed my relationship with Scorpius to model it after a couple in a tabloid or in fiction, then it wouldn't be worth keeping." She remembered all of the good times she had with Scorpius in the past year and a half of their relationship, and that was enough to keep her satisfied.
This was a thought process that Rose would never reveal to Scorpius, half out of embarrassment that she'd ever think about this and half because she did genuinely love him. These kinds of thoughts would aggravate his insecurity and she knew for a fact that he didn't deserve to listen to Rose babble about non-existent problems that wouldn't add to their already wonderful relationship. Rose remembered was grateful for what she had with a smile and went back to reading her book with a newfound appreciation for Scorpius.