Harry learned long ago never to struggle when Dudley wants to practise doing something on him. It tends to be over quicker if he just lets it happen.
So when, the summer they both turned fifteen, Dudley announced that he had a date that night, Harry knew what was coming. He was right. Dudley wanted to practise kissing, and Harry provided a convenient pair of lips.
Harry was feeling… well, depending on your interpretation, either generous or malicious that day. So when his cousin pinned him down and kissed him square on the mouth, he decided to show Dudley what it would really be like (assuming the girl didn't run screaming).
And he kissed him back.