A little ways into the episode, when Milhouse kisses Homer, he says that was the first kiss in his whole life. However, we see him kissing Samantha Stanky in the episode "Bart's Friend Falls in Love," in Season Three.
Also, Milhouse did not actually have his first kiss in this episode, despite what he said—he was kissed by Samantha Stankey in the episode "Bart's Friend Falls in Love".
https://en.wikipedia.org/wiki/Samantha_Stanky