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.
12Logically him. – V.V. – 2016-05-18T09:04:31.030
7I think "him" sounds better in that sentence. It would feel jarring to have the gender change within a single sentence. Where a person's gender really has changed (that is, when you are talking about a transgender person) you use the pronoun that applies to their current gender, but that sentence isn't implying that it is likely that he will actually become a girl, it is just saying what you'd have done if he were already a girl. – nnnnnn – 2016-05-18T09:07:28.203
4It's a supposition based on the gender of that person changing. The imagined person in that supposition is female. So it should be 'her'. – AJFaraday – 2016-05-18T15:29:56.003
"Them". I am not totally serious. – Carsten S – 2016-05-18T16:32:57.110
2@AJFaraday You seem to disagree with all of the existing answers - would you like to post an answer of your own? – Tin Wizard – 2016-05-18T16:37:17.973
3Given the current social debates in the United States, you're likely to get criticized by at least 2 separate groups for your word choice here, regardless of what you choose or what you mean to say by it. Brace thyself. – elmer007 – 2016-05-18T19:35:51.907
@elmer007 I don't really think there's any reason to bring that into this equation... the phrase is common enough that I don't think anyone really is going to take anything else from it. Nothing about this statement really reflects on the LGBT dialogue. – Catija – 2016-05-18T20:41:40.627
What a great question. I've sat here for 5 minutes arguing with myself. Personally, I would say her it fits better with the message that I want to convey, but I agree that it's probably not grammatically correct to do so. I suspect that the reality here is that the answer to the OP's question is one of art rather than structure - what would his character say? – PerryW – 2016-05-19T07:24:43.997
in Sweden we would replace all instances of him/her with the word 'hen' (not chicken..). If hen were a hen, I would have kissed hen ... makes sense – sch – 2016-05-19T11:28:39.267
@klskl ah you poor politically correct Swedes.... :-) My old (female) English teacher used to say "in English, he embraces she" meaning 'he' is used in all cases that are not explicitly female. – gbjbaanb – 2016-05-19T13:07:02.143
Highly-acclaimed writer Jonathan Franzen agrees with the pronoun suggested by @AJFaraday (See my answer, AJ.)
– Alan Carmack – 2016-05-20T02:10:27.963@7_R3X I'm trying to understand your question better. Are you wondering about transgender political correctness? If that's the case, I would say there's a big distinction between saying if he were a girl and if he became a girl. With if he were a girl, even though there is a "hypothetical gender switch" as you put it, we can't necessarily assume that the gender switch is a conscious decision made by the subject of the sentence ("he"). The hypothetical clause is conceived by the author, NOT by the subject of the sentence. – Ringo – 2016-05-25T08:05:30.233
@Ringo : My question isn't about transgender. It's about a situation where a guy impressed another by his skills. But the second guy doesn't go for males but to comment upon first guy's remarkable skills, he happen to say that "if he were a girl, I'd have kissed her/him". – 7_R3X – 2016-05-25T17:23:05.370