The correct word here is "him", as noted by other answers. But it seems to have some paradoxical feeling to it.
English is gender specific, which reflects cultural concepts and ideas passing in English-speakers' brains; it's hard to escape the question "would it be a boy or a girl that he would be kissing?"... And culturally, it is an important question: it brings about people's attitude towards transsexualism, homosexuality, and gender.
If you want to stress the cultural thing and make people aware of the dilemma, then using "her" instead might actually be a good idea! It might be regarded as grammatically incorrect, but language is a tool to express ideas and feelings.
If you want to avoid the confusion and paradoxical feeling, you may be able to use a more neutral phrasing:
If it were a girl, I would have kissed her.
This has a different feeling - a completely different hypothetical person, instead of the same person having the opposite sex, so it's "more natural"; however, it might not be fit what you're trying to express.