Watch the video I linked of "It's Not Fair." In that song, Allen is singing in quite graphic terms of a thoroughly inadequate, inconsiderate lover. "Lying in the wet spot in the middle of the bed," "spending hours giving head," references to the classic wham-bam-thank-you-ma'am, I can see where Alizée would say that she could never do anything like this because she's too modest. These are not subjects for polite conversation in mixed company, however much one may sympathize with the poor dear.
I think that's what she meant by "trashy."
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