He saying he wants the relationship to last forever and she will never have another first kiss with someone else, because he and she will be together for the rest of their lives.

Yes of course it is illogical, but that's what they want. I guess they aim to say that every kiss with her was as sweet as if it was the first kiss. So this one will be the last first kiss

Joy: Popular songs are full of contradictions in logic and in terms. It is to be expected.