Cultural thing; there's a concept of someone's "time of popularity" -- the idea that a guy will suddenly become popular with girls, get asked out, and so on, at the exact moment that he just started going out with someone. To the extent that this exists, it's likely due to a boost in self-confidence from being in a relationship, and that self-confidence being attractive, but it's become something of a meme.
(And I think they're both just being embarrassed.)