Well, personally I believe it's not about what you depict in your story, but how your narrative frames it. Depending on how you draw it, how the characters or narration talks about it and what the focus of the story is around it, the same relationship could be presented as creepy or as romantic.
For example, in the manga Card Captor Sakura, there's a Teacher and his approximately 12 year old student who are in a relationship. They are betrothed to be married. The narrative never presents this as bad. The characters are always drawn very tenderly around each other, the other characters look on them adoringly and comment on how sweet it is, nobody ever criticises it and we're never shown any ugly side to that relationship. It could be said that this depiction glorifies "grooming" by depicting a relationship between an adult man and a very underage girl as something sweet and romantic in a manga aimed at kids. Even as a fan of CCS, I think it's irresponsible of CLAMP to have included this.
Compare Scott Pilgrim Versus the World, a comic that opens up with Scott revealing that he, a 23 year old, is in a (thankfully non-sexual) relationship with a 17 year old schoolgirl. This is not presented as cute. Scott's friends are extremely critical of his choice, they're never drawn particularly romantically together and in fact the art and framing tends to draw attention to how childlike Knives Chau is and how her attempts to be an adult like the hipsters Scott hangs out with are unsettling. So that's a similar concept "an adult is dating an underage person" but it's framed critically by the narrative to say something about how Scott's immaturity is harming those around him as well as himself rather than it being glorified or glamorised as romantic like the CCS example.