Ginny knows it's wrong; knows that she shouldn't be doing this. If she is ever found out, the press will crucify her. (And it says something that she is worried about the reactions of the press, and not that of Harry and her family, doesn't it?)
She met Roger at Quidditch Camp. The doubts about her relationship with Harry had already started building at this time, and her blossoming friendship with Roger only drove the final nail into the coffin.
Ginny knows she should tell Harry about her relationship with Roger; knows she should allow both of them to move forward. But she sees the looks Harry gives good-looking men – she has always seen the looks. He may not have cheated in body like she has, but she has no doubt that he's cheated in mind. As far as she's concerned, he should tell her first. His secret is the bigger one, after all.
So she doesn't push Roger away when he kisses her, and pulls him closer instead. There hasn't been any passion in Harry's kisses for longer than she can remember (was there ever any passion there?), and Roger's kisses are nothing but passion. His kisses, unlike Harry's, feel natural.
She knows that this affair (such a dirty word for something so right) will come to light sooner or later. Until it does, she will hoard every kiss, every touch. This relationship is a secret she never wants to share. This relationship is hers.
For:
Harry Potter Spells Competition, Category Engorgio
The Not For the Faint of Heart Competition, Operation Taboo (Drabble)
The If you dare Challenge, prompt 770. Betrayed
Character Diversity Bootcamp, Prompt 38. Fake