I'm fairly sure that the kiss referred to in the third and fourth stanzas ("this other kiss") is not the final parting kiss of the guy and his ex- but rather the kiss that he has been working on with his new girlfriend. He has a chance to sleep with his ex, but he makes himself refrain, because he knows that he's got "this other kiss" waiting for him at home (ie his new girlfriend).
I'm fairly sure that the kiss referred to in the third and fourth stanzas ("this other kiss") is not the final parting kiss of the guy and his ex- but rather the kiss that he has been working on with his new girlfriend. He has a chance to sleep with his ex, but he makes himself refrain, because he knows that he's got "this other kiss" waiting for him at home (ie his new girlfriend).